Patent Publication Number: US-11022404-B2

Title: Firearm and scope alignment

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of pending U.S. patent application Ser. No. 16/288,080, filed Feb. 27, 2019, and is entitled FIREARM AND SCOPE ALIGNMENT. 
     The above-identified document is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to apparatus and methods for aligning a firearm barrel axis and a scope axis precisely in a common plane that is truly vertical, and aligning the vertical and horizontal scope reticles to be truly vertical and horizontal. 
     BACKGROUND 
     Precisely vertical and horizontal rifle scope crosshairs, or reticles, become increasingly important to shot accuracy as the downrange distance increases. A slight cant, or angle error, in the crosshairs can cause noticeable error or missed shots at long distances. A cant of five degrees may cause an error of 3.7 inches at 300 yards. 
     Stated another way, the firearm barrel axis, along which the projectile travels, must lie in a common vertical plane with the scope axis, along which the shooter sights the target. However, when the firearm is properly sighted in, the barrel axis is not parallel to the scope axis. The barrel axis must be tilted with the muzzle up (elevation) in order to counteract gravitational drop of the projectile during its flight time to the target. However, the scope axis is usually truly horizontal. When the barrel and scope axes lie in a non-vertical plane, the barrel axis elevation angle causes the projectile to miss low to the right or left of the target. 
     Numerous devices have been developed with the goal of aligning a firearm barrel axis and a scope axis precisely in a common vertical plane and aligning the vertical and horizontal scope reticles to be truly vertical and horizontal. Many of these devices incorporate spirit levels. Some examples include the Weaver Crosshair Leveling Kit, the Straight Shot Segway Reticle Kit and the Wheeler Pro Reticle Leveling Kit. There are at least two shortcomings with devices such as these: 1) inadequate means for mounting the devices to the firearm or scope and 2) low precision fabrication of the devices, in particular low sensitivity spirit levels. 
     The means for mounting the devices to the firearm or scope may be inadequate for several reasons. Devices may make contact with firearm or scope features or surfaces which are not reliably and precisely oriented with respect to the barrel axis, the scope axis, and/or the vertical plane for alignment. Devices may be fabricated with compliant parts, such as a magnetic pad, for primary contact with the firearm or scope. Devices may have relatively small contact areas or contact lengths with the firearm or scope features. 
     Devices may be fabricated with conventional industry tolerances for production parts, or in some instances tolerances that may be wider than industry standard. In an assembly, the tolerances may stack up unfavorably, resulting in low precision. 
     There is a need for apparatus and methods which overcome these drawbacks by incorporating means for mounting the apparatus to reliable precision machined surfaces of the firearm or scope, which surfaces have known, consistent orientations relative to the barrel axis, the scope axis, and/or the vertical plane for alignment. There is also a need for apparatus and methods with high precision, for example components fabricated and assembled to gage makers tolerances, and high sensitivity spirit levels. 
     SUMMARY 
     The various systems and methods of the present technology have been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available firearm/scope alignment and leveling technologies. The systems and methods of the present technology may provide enhanced means for mounting the associated apparatus to a firearm or scope and enhanced precision of parts, assemblies, and methods of use. 
     To achieve the foregoing, and in accordance with the technology as embodied and broadly described herein, in an aspect of the technology, a system for leveling a firearm receiver, wherein the receiver includes a central longitudinal axis, a cavity, an ejection window, and a magazine window, wherein the cavity extends longitudinally into a back end of the receiver and includes a first planar datum surface that is parallel to the central longitudinal axis, wherein the ejection window extends into the cavity through an upper portion of the receiver between an ejection window front wall and an ejection window back wall, wherein the magazine window extends into the cavity through a bottom side of the receiver between a magazine window front wall and a magazine window back wall, the system includes: a bar assembly including a bar and a first level, wherein the bar assembly is removably connectable to the receiver; wherein the bar extends between a front end and an opposite back end, wherein the bar includes a second planar datum surface that extends between the front and back ends; wherein the first level is fixed to the bar and is level with respect to the second planar datum surface; wherein when the bar assembly is connected to the receiver, the first and second planar datum surfaces are in direct contact, the bar front end is in front of the ejection window front wall or the magazine window front wall, and the bar back end is behind the ejection window back wall or the magazine window back wall. 
     Embodiments of this aspect may include one or more of the following attributes. The first planar datum surface extends between a first front edge and an opposite first back edge; wherein the second planar datum surface extends between a second front edge and an opposite second back edge; wherein, when the bar assembly is connected to the receiver, the second front edge is in front of the first front edge, and the second back edge is behind the first back edge. When the bar assembly is connected to the receiver, the second planar datum surface is in direct contact with at least 40% of the first planar datum surface. The first level is a bullseye spirit level, wherein the first level is omnidirectionally level with respect to the second planar datum surface. The bar assembly includes a second level that is fixed to the bar and is level with respect to the second planar datum surface, wherein the second level is a tube spirit level; wherein when the bar assembly is connected to the receiver, the second level is elongated along a direction that is perpendicular to the receiver central longitudinal axis. The system further including: a plate assembly including a lower plate, an upper plate, and a windage fine adjustment mechanism; wherein the lower plate is removably connectable to a main column of a firearm bench rest; wherein the upper plate is removably connectable to the receiver; wherein the windage fine adjustment mechanism is actuatable to pivot the upper plate right or left relative to the lower plate about a pivot axis. When the receiver is connected to the upper plate, the receiver central longitudinal axis intersects the pivot axis. 
     In another aspect of the technology, a system for leveling a firearm receiver, wherein the receiver includes a central longitudinal axis and a cavity, wherein the cavity extends longitudinally into a back end of the receiver and includes a first planar datum surface that is parallel to the central longitudinal axis, wherein the first planar datum surface extends between a first front edge and an opposite first back edge, wherein a first distance extends parallel to the central longitudinal axis between the first front and back edges, the system includes: a bar assembly including a bar and a first level, wherein the bar assembly is removably connectable to the receiver; wherein the bar includes a second planar datum surface that extends between a second front edge and an opposite second back edge; wherein the first level is fixed to the bar and is level with respect to the bar second planar datum surface; wherein when the bar assembly is connected to the receiver, the first and second planar datum surfaces are in direct contact, a second distance extends parallel to the central longitudinal axis between the second front and back edges, wherein the second distance is at least 50% of the first distance. 
     Embodiments of this aspect may include one or more of the following attributes. When the bar assembly is connected to the receiver, the second front edge is in front of the first front edge, and the second back edge is behind the first back edge. When the bar assembly is connected to the receiver, the second planar datum surface is in direct contact with at least 40% of the first planar datum surface. The first level is a bullseye spirit level, wherein the first level is omnidirectionally level with respect to the second planar datum surface. The bar assembly includes a second level that is fixed to the bar and is level with respect to the second planar datum surface, wherein the second level is a tube spirit level; wherein when the bar assembly is connected to the receiver, the second level is elongated along a direction that is perpendicular to the receiver central longitudinal axis. The system further including: a plate assembly including a lower plate, an upper plate, and a windage fine adjustment mechanism; wherein the lower plate is removably connectable to a main column of a firearm bench rest; wherein the upper plate is removably o connectable to the receiver; wherein the windage fine adjustment mechanism is actuatable to pivot the upper plate right or left relative to the lower plate about a pivot axis. When the receiver is connected to the upper plate, the receiver central longitudinal axis intersects the pivot axis. 
     In yet another aspect of the technology, a method of leveling a firearm receiver, wherein the receiver includes a central longitudinal axis, a cavity, an ejection window, and a magazine window, wherein the cavity extends longitudinally into a back end of the receiver and includes a first planar datum surface that is parallel to the central longitudinal axis, wherein the ejection window extends into the cavity through an upper portion of the receiver between an ejection window front wall and an ejection window back wall, wherein the magazine window extends into the cavity through a bottom side of the receiver between a magazine window front wall and a magazine window back wall, the method includes the steps of: providing a bar assembly including a bar and a first level, wherein the bar extends between a front end and an opposite back end, and includes a second planar datum surface that extends between the front and back ends, wherein the first level is fixed to the bar and is level with respect to the second planar datum surface, wherein the first level includes a first bubble; coupling the bar assembly to the receiver so that the first and second planar datum surfaces are in direct contact, the bar front end is in front of the ejection window front wall or the magazine window front wall, and the bar back end is behind the ejection window back wall or the magazine window back wall; and adjusting the orientation of the receiver so that the first bubble is centered in the first level. 
     Embodiments of this aspect may include one or more of the following attributes. The method, further including the steps of: coupling a scope to the receiver, wherein the scope includes a vertical reticle; providing a true vertical datum downrange of the receiver; and after adjusting the orientation of the receiver so that the first bubble is centered in the first level, adjusting the orientation of the scope to align the vertical reticle to the true vertical datum while maintaining the first bubble centered in the first level; and fixing the scope to the receiver while maintaining the first bubble centered in the first level and the vertical reticle aligned to the true vertical datum. The first planar datum surface extends between a first front edge and an opposite first back edge, wherein the second planar datum surface extends between a second front edge and an opposite second back edge, the method further including the steps of: coupling the bar assembly to the receiver so that the second front edge is in front of the first front edge, and the second back edge is behind the first back edge. The method, further including the steps of: coupling the bar assembly to the receiver so that the second planar datum surface is in direct contact with at least 40% of the first planar datum surface. The method, further including the steps of: providing a plate assembly including a lower plate, an upper plate, and a windage fine adjustment mechanism, wherein the windage fine adjustment mechanism is actuatable to pivot the upper plate right or left relative to the lower plate about a pivot axis; coupling the lower plate to a main column of a firearm bench rest; and coupling the upper plate to the receiver; wherein adjusting the orientation of the receiver so that the first bubble is centered in the first level includes adjusting the elevation of the receiver central longitudinal axis by adjusting the bench rest, and adjusting the windage of the receiver central longitudinal axis by adjusting the windage fine adjustment mechanism. The first level is a bullseye spirit level, wherein adjusting the orientation of the receiver so that the first bubble is centered in the first level includes omnidirectionally adjusting the orientation of the receiver, the method further including the steps of: coupling a scope to the receiver, wherein the scope includes a vertical reticle; providing a true vertical datum downrange of the receiver; and after adjusting the orientation of the receiver so that the first bubble is centered in the first level, adjusting the orientation of the scope to align the vertical reticle to the true vertical datum while maintaining the first bubble centered in the first level; and fixing the scope to the receiver while maintaining the first bubble centered in the first level and the vertical reticle aligned to the true vertical datum. 
     These and other features and advantages of the present technology will become more fully apparent from the following description and appended claims, or may be learned by the practice of the technology as set forth hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the technology will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only exemplary embodiments and are, therefore, not to be considered limiting of the scope of the technology, the exemplary embodiments will be described with additional specificity and detail through use of the accompanying drawings in which: 
         FIG. 1  is a perspective view of a first assembly, referred to as a reticle bar; 
         FIG. 2  is another perspective view of the first assembly of  FIG. 1  from a different direction; 
         FIG. 3  is a top view of the first assembly of  FIG. 1 ; 
         FIG. 4  is a bottom view of the first assembly of  FIG. 1 ; 
         FIG. 5  is a front view of the first assembly of  FIG. 1 ; 
         FIG. 6  is a back view of the first assembly of  FIG. 1 ; 
         FIG. 7  is a left view of the first assembly of  FIG. 1 ; 
         FIG. 8  is a right view of the first assembly of  FIG. 1 ; 
         FIG. 9  is an exploded perspective view of the first assembly of  FIG. 1 ; 
         FIG. 10  is another exploded perspective view of the first assembly of  FIG. 1  from a different direction; 
         FIG. 11  is a perspective view of a receiver; 
         FIG. 12  is another perspective view of the receiver of  FIG. 11  from a different direction; 
         FIG. 13  is another perspective view of the receiver of  FIG. 11  from a different direction; 
         FIG. 14  is another perspective view of the receiver of  FIG. 11  from a different direction; 
         FIG. 15  is another perspective view of the receiver of  FIG. 11  from a different direction; 
         FIG. 16  is another perspective view of the receiver of  FIG. 11  from a different direction; 
         FIG. 17  is a cross-sectional view of a top half of the receiver of  FIG. 11 , taken along section line  17 - 17  of  FIG. 18 ; 
         FIG. 18  is a right view of the receiver of  FIG. 11 ; 
         FIG. 19  is a cross-sectional view of a bottom half of the receiver of  FIG. 11 , taken along section line  19 - 19  of  FIG. 18 ; 
         FIG. 20  is a perspective view of the first assembly of  FIG. 1  operatively assembled in the receiver of  FIG. 11 ; 
         FIG. 21  is a perspective view of the first assembly and receiver of  FIG. 20  from a different direction; 
         FIG. 22  is a front view of the first assembly and receiver of  FIG. 20 ; 
         FIG. 23  is a right view of the first assembly and receiver of  FIG. 20 ; 
         FIG. 24  is a cross-sectional view of the first assembly and receiver of  FIG. 20 , taken along section line  24 - 24  of  FIG. 22 ; 
         FIG. 25  is a cross-sectional view of the first assembly and receiver of  FIG. 20 , taken along section line  25 - 25  of  FIG. 23 ; 
         FIG. 26  is a cross-sectional view of the first assembly and receiver of  FIG. 20 , taken along section line  26 - 26  of  FIG. 23 ; 
         FIG. 27  is a cross-sectional view of the first assembly and receiver of  FIG. 20 , taken along section line  27 - 27  of  FIG. 23 ; 
         FIG. 28  is a cross-sectional view of the first assembly and receiver of  FIG. 20 , taken along section line  28 - 28  of  FIG. 23 ; 
         FIG. 29  is a cross-sectional view of the first assembly and receiver of  FIG. 20 , taken along section line  29 - 29  of  FIG. 23 ; 
         FIG. 30  is a cross-sectional view of the first assembly and receiver of  FIG. 20 , taken along section line  30 - 30  of  FIG. 23 ; 
         FIG. 31  is a cross-sectional view of the first assembly and receiver of  FIG. 20 , taken along section line  31 - 31  of  FIG. 23 ; 
         FIG. 32  is a right detail view of a portion of the first assembly of  FIG. 1  operatively arranged adjacent to a bolt stop/release; 
         FIG. 33  is a bottom detail view of a portion of the first assembly and bolt stop/release of  FIG. 32 ; 
         FIG. 34  is a right detail view of a portion of the first assembly of  FIG. 1  operatively arranged adjacent to another bolt stop/release; 
         FIG. 35  is a bottom detail view of a portion of the first assembly and bolt stop/release of  FIG. 34 ; 
         FIG. 36  is a perspective view of a plate sub-assembly of a second assembly operatively assembled to a rifle bench rest, an adapter block base of the second assembly operatively assembled to the plate sub-assembly, three adapter blocks of the second assembly, and two more main columns for interchangeable use in the rifle bench rest, the second assembly also referred to as a sighting holder; 
         FIG. 37  is an exploded perspective view of the plate sub-assembly and adapter block base of  FIG. 36 ; 
         FIG. 38  is a top view of the plate sub-assembly and adapter block base of  FIG. 36 ; 
         FIG. 39  is a front view of the plate sub-assembly and adapter block base of  FIG. 38 , with one of the adapter blocks of  FIG. 36  operatively assembled to the adapter block base; 
         FIG. 40  is a cross-sectional view of the plate sub-assembly, adapter block base, and adapter block of  FIG. 39 , taken along section line  40 - 40  of  FIG. 39 ; 
         FIG. 41  is a perspective view of the plate sub-assembly, rifle bench rest, and adapter block base of  FIG. 36 , with another one of the adapter blocks of  FIG. 36  operatively assembled to the adapter block base; 
         FIG. 42  is a perspective view of the plate sub-assembly, rifle bench rest, adapter block base, and adapter block of  FIG. 41  operatively assembled to the first assembly and receiver of  FIG. 20 ; 
         FIG. 43  is a top view of the plate sub-assembly, rifle bench rest, adapter block base, adapter block, first assembly, and receiver of  FIG. 42 ; 
         FIG. 44  is another perspective view of the plate sub-assembly, rifle bench rest, adapter block base, adapter block, first assembly, and receiver of  FIG. 42  from a different direction; 
         FIG. 45  is a back view of the plate sub-assembly, rifle bench rest, adapter block base, adapter block, first assembly, and receiver of  FIG. 42  operatively assembled to a rifle scope, shown schematically; 
         FIG. 46  is a perspective view of the plate sub-assembly, adapter block base, and adapter block of  FIG. 38  operatively assembled to the rifle bench rest of  FIG. 36 ; 
         FIG. 47  is a back view of the plate sub-assembly, adapter block base, adapter block, and rifle bench rest of  FIG. 46 ; 
         FIG. 48  is a top view of the plate sub-assembly, adapter block base, adapter block, and rifle bench rest of  FIG. 46 ; 
         FIG. 49  is a perspective view of the plate sub-assembly, rifle bench rest, and adapter block base of  FIG. 36 , with yet another one of the adapter blocks of  FIG. 36  operatively assembled to the adapter block base; 
         FIG. 50  is a perspective view of the plate sub-assembly, rifle bench rest, adapter block base, and adapter block of  FIG. 49 , with a rifle scope operatively assembled to the adapter block; 
         FIG. 51  is a back view of the plate sub-assembly, rifle bench rest, adapter block base, adapter block, and rifle scope of  FIG. 50 ; 
         FIG. 52  is a side view of the plate sub-assembly, rifle bench rest, adapter block base, adapter block, and rifle scope of  FIG. 50 ; 
         FIG. 53  is a bottom view of an AR action; 
         FIG. 54  is an oblique view of the AR action of  FIG. 53  coupled to a scope, the AR action being coupled to a third assembly, referred to as an AR sighting holder, the third assembly coupled to a rifle bench rest; 
         FIG. 55  is an oblique detail view of a portion of the AR action and third assembly of  FIG. 54 ; 
         FIG. 56  is an oblique detail view of a portion of the AR action, third assembly, and rifle bench rest of  FIG. 54 ; 
         FIG. 57  is an oblique view of the AR action, scope, third assembly, and rifle bench rest of  FIG. 54 , the AR action fully coupled to the third assembly; 
         FIG. 58  is an oblique detail view of a portion of the third assembly and rifle bench rest of  FIG. 54 ; 
         FIG. 59  is an oblique detail view of a portion of the AR action, third assembly, and rifle bench rest of  FIG. 54 ; 
         FIG. 60  is another oblique view of the AR action, scope, third assembly, and rifle bench rest of  FIG. 54 ; and 
         FIG. 61  is yet another oblique view of the AR action, scope, third assembly, and rifle bench rest of  FIG. 54 . 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the technology will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. It will be readily understood that the components of the technology, as generally described and illustrated in the figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the apparatus, system, and method is not intended to limit the scope of the invention, as claimed, but is merely representative of exemplary embodiments of the technology. 
     The phrases “connected to,” “coupled to” and “in communication with” refer to any form of interaction between two or more entities, including mechanical, electrical, magnetic, electromagnetic, fluid, and thermal interaction. Two components may be functionally coupled to each other even though they are not in direct contact with each other. The term “abutting” refers to items that are in direct physical contact with each other, although the items may not necessarily be attached together. The phrase “fluid communication” refers to two features that are connected such that a fluid within one feature is able to pass into the other feature. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
     Standard firearm planes of reference, directional terms, and descriptive terminology are employed in this specification with their ordinary and customary meanings. “Front” or “muzzle,” “rear” or “back” or “breech,” “left,” “right,” “top” or “up,” and “bottom” or “down” are defined from the point of view of a shooter in a conventional shooting position relative to an apparatus or a part. “Downrange” means in front of the apparatus at a distance representative of a planned distance to target. A firearm has a barrel axis, or shooting axis, which is the central longitudinal axis of the barrel, along which a bullet travels as it is fired from the firearm. A scope has a scope axis, or sighting axis, which is the central longitudinal axis of the scope, along which a shooter sights on a target. When the firearm and scope are properly aligned, the barrel axis and the scope axis lie in a common vertical plane. Typically, the scope axis is above the barrel axis. 
     “Gage makers tolerance,” abbreviated as “GMT,” is defined in this specification as a tolerance much smaller than conventional industry tolerances for production parts. GMTs are conventionally used in fabrication of the gages used to inspect production parts. ASME B89 is one example of a standard for dimensional metrology which sets forth specification of dimensional measuring instruments and gages for measuring various geometrical characteristics such as lengths, plane surfaces, angles, circles, cylinders, cones, spheres, and tori. 
     Referring to  FIGS. 1-10 , a first assembly  100  may include a base  102 , a bullseye level  104 , a tube level  106 , a roller  108 , a screw  114 , and a set screw  120 . Three rollers  108 ,  110 ,  112 ; three screws  114 ,  116 ,  118 ; and two set screws  120 ,  122  are shown. The first assembly  100  may be referred to as a reticle bar, reticle assembly, or bar assembly. 
     The base  102  may be a long, thin, substantially rectangular bar that is elongated between a front end  130  and a back end  132 . The base  102  has a top side  134 , a bottom side  136 , a right side  138 , and a left side  140 . First, second, and third holes  142 ,  144 ,  146  may extend through the base  102  between the top and bottom sides  134 ,  136 . The holes  142 ,  144 ,  146  may be centered in the right-left width of the base  102 . The first hole  142  may be near the front end  130 . The second and third holes  144 ,  146  may be close together, about halfway between the front and back ends  130 ,  132 , with the second hole closer to the front end and the third hole closer to the back end. The holes  142 ,  144 ,  146  may be internally threaded. A triangular group of three holes  148 ,  150 ,  152  may extend through the base  102  between the top and bottom sides  134 ,  136 . The holes  148 ,  150 ,  152  may be close together near the back end  132 , with the hole  148  centered in the right-left width of the base  102  and closer to the front end  130  and the holes  150 ,  152  side by side closer to the back end. The hole  150  may be closer to the left side  140  and the hole  152  may be closer to the right side  138 . A pair of holes  154 ,  156  may extend through the base  102  between the top and bottom sides  134 ,  136 . The holes  154 ,  156  may be closer to the back end than the holes  150 ,  152 . The hole  154  may be closer to the left side  140  and the hole  156  may be closer to the right side  138 . A transverse groove  158  may extend into the base  102  from the top side  134  and across the base between the right and left sides  138 ,  140 . The groove  158  may have a circular cross-sectional shape when viewed in a right or left view. The center of the circular cross-sectional shape may be recessed below the top side  134  so that the groove  158  is undercut ( FIGS. 7, 8, 24 ). A longitudinal notch  160  may extend into the base  102  along the edge between the bottom and right sides  136 ,  138 . The front end of the notch  160  may be close to the front side of the hole  144 . The back end of the notch  160  may be between the hole  146  and the hole  148 . The notch  160  may have a rectangular cross-sectional shape when viewed in a front or back view. Another longitudinal notch  162  may extend into the base  102  along the edge between the bottom and left sides  136 ,  140 . The notch  162  may be a mirror image of the notch  160 . 
     The bullseye level  104  may be a cylindrical part with a top end  164  and a bottom end  166 . The bullseye level  104  may be referred to as a circular spirit level or an omnidirectional spirit level. An enlarged flange  168  may extend around the bottom end. A triangular group of three holes  170 ,  172 ,  174  may extend through the flange  168  between the top and bottom ends  164 ,  166 . The bullseye level  104  includes a fluid reservoir  176  with a clear wall across the top end. The fluid reservoir includes a bubble of air or other gas that is movable within the fluid. The clear top wall may include indicia, such as concentric circular marks, to aid the user in centering the bubble and thus leveling the bullseye level  104 . The clear top wall is preferably made of glass. Geier &amp; Bluhm of New York manufactures a bullseye level that the inventor finds suitable. 
     The tube level  106  may be an elongated clear part that extends between a right end  180  and a left end  182 . While the tube level  106  may appear to be a cylindrical part, it may actually have an oval shape, a barrel shape, or another curved shape of its side wall in a front or back view. The curvature may be subtle, in other words, it may have a large radius of curvature. The tube level  106  is a fluid reservoir with a bubble of air or other gas that is movable within the fluid. The tube level  106  may include indicia, such as transverse lines, to aid the user in centering the bubble and thus leveling the tube level. The tube level  106  is preferably made of glass. W. A. Moyer of Kansas manufactures a tube level that the inventor finds suitable. 
     The “sensitivity” of a spirit level refers to how easily the bubble moves within the fluid reservoir when the spirit level is tilted. Greater sensitivity equates to a more precise spirit level. Sensitivity may be measured in millimeters per meter (mm/m) or arcminute (arcmin). The standard for most spirit levels on the market is a sensitivity of 10 mm/m. Preferably, the bullseye level  104  and the tube level  106  are high precision parts with sensitivities less than 10 mm/m, for example 5 mm/m, 2 mm/m, or 1 mm/m. 
     The roller  108  may be a ball nose spring plunger or other compliant component. The roller  108  may be externally threaded. The rollers  110 ,  112  may be identical to the roller  108 . 
     The screw  114  may be a socket head cap screw. The screws  116 ,  118  may be identical to the screw  114 . 
     The set screw  120  may be a cup point socket set screw. The set screw  122  may be identical to the set screw  120 . 
     The first assembly  100  may be operatively assembled by inserting the roller  108  into the hole  142  of the base  102  with the ball nose protruding from the top side  134 ; inserting the roller  110  into the hole  144  with the ball nose protruding from the top side  134 ; inserting the roller  112  into the hole  146  with the ball nose protruding from the top side  134 ; coupling the bullseye level  104  to the base  102  by inserting the screw  114  through the hole  170  and into the hole  148 , inserting the screw  116  through the hole  172  and into the hole  150 , and inserting the screw  118  through the hole  174  and into the hole  152 ; and coupling the tube level  106  to the base  102  by sliding the tube level into the groove  158  from the right or left side  138 ,  140 , inserting the set screw  120  into the hole  154  to press against the bottom side of the tube level, and inserting the set screw  122  into the hole  156  to press against the bottom side of the tube level. Inserting the rollers  108 ,  110 ,  112  into the corresponding holes  142 ,  144 ,  146  may involve threading the rollers into the holes or press-fitting the rollers into the holes. Coupling the bullseye level  104  and/or the tube level  106  to the base  102  may include precisely leveling the bullseye level and/or the tube level to the bottom side  136  of the base  102 , for example by adjusting the screws  114 ,  116 ,  118  and/or set screws  120 ,  122  to center the bubble(s) to GMT while the bottom side  136  rests upon a precision datum surface such as a calibrated granite surface plate. A surface plate is a solid, flat plate commonly used as the main horizontal reference plane for precision inspection, layout, and tooling setup. 
     When the first assembly  100  is operatively assembled, the bullseye level  104  and the tube level  106  may be precisely leveled with respect to the bottom side  136  of the base  102 . The tops of the rollers  108 ,  110 ,  112  may be precisely positioned at a specific distance from the bottom side  136 . Thus, the bottom side  136  may function as a primary planar datum surface of the first assembly  100 . 
     Referring to  FIGS. 11-19 , a receiver  200  is shown. The illustrated receiver  200  is a flat bottomed Mod-70 which includes certain features that are common to many receivers. Only those features which interact with components of the current technology will be described herein. 
     The receiver  200  extends between a front end  210  and a back end  212 . A central longitudinal axis  214  extends between the front and back ends  210 ,  212 . When the receiver  200  is operatively assembled into a firearm, the axis  214  extends along the center of the barrel. Thus the axis  214  is the barrel axis or shooting axis of the receiver  200 . The exterior of the receiver  200  includes a flat bottom surface  216  with an internally threaded hole  218  that extends up through the bottom surface  216  into the receiver  200  near the front end  210 . The hole  218  may be referred to as a front guard screw hole. The interior of the receiver  200  includes a longitudinal cavity  220 . The cavity includes a front right lower planar surface  222 , a rear right lower planar surface  224 , and a left lower planar surface  226 . In the example shown, the front right lower planar surface  222  has an area of 1007 mm 2  and a 135 mm length measured parallel to the axis  214 , the rear right lower planar surface  224  has an area of 142 mm 2  and a 22 mm length parallel to the axis  214 , and the left lower planar surface  226  has an area of 808 mm 2  and a 167 mm length parallel to the axis  214 . The lower planar surfaces  222 ,  224 ,  226  may be coplanar, and may be referred to as lower rails. The lower planar surfaces  222 ,  224 ,  226  may be parallel to the axis  214 . The lower planar surfaces  222 ,  224 ,  226  are reliable precision machined surfaces which are excellent primary datum features for the disclosed technology. Taken together, the lower planar surfaces  222 ,  224 ,  226  may be treated as a single lower planar datum surface  221 . In the example shown, the lower planar datum surface  221  has an area of 1957 mm 2  and a 169 mm overall length measured parallel to the axis  214 . The cavity  220  includes a front right upper planar surface  228 , a rear right upper planar surface  230 , and a left upper planar surface  232 . The upper planar surfaces  228 ,  230 ,  232  may be coplanar, and may be referred to as upper rails. The upper planar surfaces  228 ,  230 ,  232  are reliable precision machined surfaces which are excellent primary datum features for the disclosed technology. Taken together, the upper planar surfaces  228 ,  230 ,  232  may be treated as a single upper planar datum surface  227 . The upper planar surfaces  228 ,  230 ,  232  may be parallel to the lower planar surfaces  222 ,  224 ,  226 . Taken together, the lower and upper rails may be referred to as a bolt raceway. The cavity  220  includes a first cylindrical portion  234  which extends rearwardly into the front end  210  and a second cylindrical portion  236  which extends rearwardly from the first cylindrical portion  234  and concentric with the first cylindrical portion. The first cylindrical portion  234  receives a barrel (indicated in dashed lines in  FIG. 43 ). The diameter of the second cylindrical portion  236  may be greater than the diameter of the first cylindrical portion  234 , so that a rear-facing annular wall  238  is formed between the first and second cylindrical portions  234 ,  236 . The wall  238  may be referred to as a chamber breech. The right side of the second cylindrical portion  236  may extend rearwardly past the wall  238  to intersect the front right lower planar surface  222  and the front right upper planar surface  228  to form outer edges of the planar surfaces. An upper concave surface  240  extends rearwardly from the second cylindrical portion  236 . The upper concave surface  240  may be a section of a cylinder, and may be concentric with the first and second cylindrical portions  234 ,  236 . The diameter of the upper concave surface  240  may be less than the diameter of the second cylindrical portion  236 , so that a front-facing wall  242  is formed between the second cylindrical portion and the upper concave surface. The upper concave surface  240  may intersect the upper planar surfaces  228 ,  230 ,  232  to form inner edges of the upper planar surfaces. A front lower concave surface  244  extends rearwardly from the second cylindrical portion  236 . The front lower concave surface  244  may be a section of the same cylinder as the upper concave surface  240 . Thus, the wall  242  may exist between the second cylindrical portion  236  and the front lower concave surface  244 . The wall  242  may be the front boundary of the front right upper planar surface  228  and the left lower planar surface  226 . The front lower concave surface  244  may intersect the lower planar surfaces  222 ,  226  to form inner edges of the lower planar surfaces. A longitudinal window  246  extends upwardly into the receiver  200  to intersect the cavity  220  to the rear of the front lower concave surface  244 . The window  246  may be referred to as a magazine window or a feed window. A rear lower concave surface  248  extends rearwardly from the window  246 . The rear lower concave surface  248  may be a section of the same cylinder as the upper concave surface  240 . The rear lower concave surface  248  may intersect the lower planar surfaces  222 ,  224 ,  226  to form inner edges of the lower planar surfaces. A longitudinal window  250  extends into the top and right sides of the receiver  200  to intersect the cavity  220 . The window  250  may be referred to as an ejection window or ejection port. Referring to  FIGS. 17-19 , the front end or wall of the window  250  may be slightly in front of the front end or wall of the window  246 . The top portion between the front end  210  and the window  250  may be referred to as a front bridge or front arch. The rear end or wall of the window  250  may be slightly in front of the rear end or wall of the window  246 . The top portion behind the window  250  may be referred to as a rear bridge or rear arch. A rear right concave surface  252  extends rearwardly from the rear end of the window  250 . The rear right concave surface  252  intersects the rear right lower planar surface  224  and the rear right upper planar surface  230  to form outer edges of the planar surfaces. The rear right concave surface  252  may be a section of the same cylinder as the second cylindrical portion  236 . A front left concave surface  254  extends rearwardly from the wall  242  past the rear end of the window  246 , intersects the left upper planar surface  232  and the left lower planar surface  226  to form outer edges of the planar surfaces, and may be a section of the same cylinder as the first cylindrical portion  234 . A window  256  extends into the left side of the receiver  200  to intersect the cavity  220  and form the rear end of the front left concave surface  254 . A rear left concave surface  258  extends rearwardly from the window  256 , intersects the left upper planar surface  232  and the left lower planar surface  226  to form outer edges of the planar surfaces, and may be a section of the same cylinder as the first cylindrical portion  234 . 
     Referring to  FIGS. 20-31 , the first assembly  100  may be operatively assembled to the receiver  200  by inserting the front end  130  of the base  102  of the first assembly  100  into the back end of the cavity  220  of the receiver  200  and advancing the first assembly  100  within the cavity  220  so that the bottom side  136  directly contacts at least one, and preferably all, of the lower planar surfaces  222 ,  224 ,  226 ; the top side  134  faces the upper planar surfaces  228 ,  230 ,  232 ; the roller  108  contacts the upper concave surface  240  in front of the window  250 ; and at least one roller  110 ,  112  contacts the upper concave surface  240  to the rear of the window  246 . The front end  130  of the base  102  may contact the chamber breech, in other words, the wall  238  or the back or breech end of a barrel coupled to the receiver  200 . 
     When the first assembly  100  is operatively assembled to the receiver  200 , the bottom side  136  is directly adjacent to the front ends of the lower planar surfaces  222 ,  226  and the rear ends of the lower planar surfaces  222 ,  224 . The front end of the bottom side  136  is in front of the front end of the window  246 , may be in front of the front end of the window  250 , and may be in front of the front ends of the lower planar surfaces  222 ,  226 . The rear end of the bottom side  136  is behind the rear end of the window  250 , may be behind the rear end of the window  246 , and may be behind the rear ends of the lower planar surfaces  222 ,  224 . The right and left sides  138 ,  140  are directly adjacent to the outer edges of the lower planar surfaces  222 ,  224 ,  226 . Thus, surface contact between the bottom side  136  and the lower planar datum surface  221  is maximized within the constraint of physically sliding the first assembly  100  into the cavity  220  from the rear. Said another way, the bottom side  136  contacts substantially the full length of the lower planar datum surface  221  measured parallel to the axis  214 . From a metrology point of view, there will be at least three points of contact between the bottom side  136  and the lower planar datum surface  221 . The three points of contact establish a primary datum plane  260  common to the first assembly  100  and the receiver  200 . The primary datum plane  260  may be referred to as a horizontal datum plane. While this description is made in the context of a continuous flat planar bottom side  136 , it is contemplated that the bottom side  136  may instead comprise discontinuous patches, or points, for contacting the lower planar datum surface  221 . Taken together, the discontinuous patches or points may function as a primary planar datum surface. Whether the bottom side  136  is continuous or discontinuous, the bottom side  136  preferably directly contacts the full length of the lower planar datum surface  221  measured parallel to the axis  214 . 
     In the example shown, when the first assembly  100  is operatively assembled to the receiver  200  as shown in  FIGS. 20-31 , on an area basis, the bottom side  136  contacts 833 mm 2  (43%) of the lower planar datum surface  221 , including contacting 379 mm 2  (38%) of the front right lower planar surface  222 , 26 mm 2  (18%) of the rear right lower planar surface  224 , and 428 mm 2  (53%) of the left lower planar surface  226 . It is contemplated that the bottom side  136  may be in direct contact with at least 40% of the area of the lower planar datum surface  221 , the front right lower planar surface  222 , or the left lower planar surface  226 . On a length basis, parallel to the axis  214 , the bottom side  136  contacts the full length of the lower planar datum surface  221 . However, it is contemplated that the bottom side  136  may contact at least 50% of the length of the lower planar datum surface  221 , the front right lower planar surface  222 , or the left lower planar surface  226 . 
     When the first assembly  100  is operatively assembled to the receiver  200 , the bullseye level  104  and the tube level  106  are behind the back end of the receiver  200 . The tube level  104  is oriented with its length, from right to left, perpendicular to the receiver axis  214 . 
     The roller  108  contacts the upper concave surface  240  in front of the window  250 . At least one roller  110 ,  112  contacts the upper concave surface  240  to the rear of the window  246 . Two rear rollers  110 ,  112  are provided to accommodate different size receivers  200 . Preferably, the rollers  108 ,  110 ,  112  touch intact portions of the upper concave surface  240  away from any holes or other interruptions of the upper concave surface. Preferably, the rollers  108 ,  110 ,  112  are compliant, for example biased to extend up from the top side  134  of the base  102 . In this arrangement, compliant rollers  108 ,  110 ,  112  may deflect downward as they contact the upper concave surface  240 . The downward deflection may serve at least two purposes: to automatically accommodate dimensional variations between the upper concave surface  240  and the primary datum plane  260  in individual receivers, and to automatically seek the highest location against the upper concave surface due to the bias which urges the rollers  108 ,  110 ,  112  up toward their undeflected state. As the rollers  108 ,  110 ,  112  seek the highest location against the upper concave surface  240 , the first assembly  100  may be urged right or left within the cavity  220  as a result, thus automatically aligning the first assembly  100  along the interior crest of the upper concave surface  240 . From a metrology point of view, each roller will have a point of contact with the upper concave surface  240 . Two points of contact establish a secondary datum line  262  along the interior crest of the upper concave surface  240 , which establishes a secondary datum plane  264  that contains the secondary datum line  262  and is perpendicular to the primary datum plane  260 . The secondary datum plane  264  may be referred to as a vertical datum plane. In the nominal design of the receiver  200 , the secondary datum plane  264  may also contain the axis  214 . However, it is contemplated that the first assembly  100  may rest asymmetrically within the cavity  220 , in other words, offset to the right or left. This may cause the secondary datum plane  264  to be left or right of the axis  214 . Alternatively, even if the first assembly  100  rests to the right or left, the rollers  108 ,  110 ,  112  may be positioned relative to the base  102  to counteract this eccentricity, so that the secondary datum plane  264  may contain the axis  214 . 
     When the first assembly  100  is operatively assembled to the receiver  200 , various parts normally associated with the receiver  200  may remain assembled to the receiver  200  without interfering with the first assembly  100 . For example, the trigger assembly and/or ejector may remain assembled to the receiver  200  without pushing on the first assembly  100 . Preferably, the magazine well assembly and stock are disassembled from the receiver  200  before the first assembly  100  is operatively assembled to the receiver  200 . 
     Referring to  FIGS. 32-33 , side and bottom views show the first assembly  100  operatively arranged adjacent to a bolt stop/release  202  characteristic of a Winchester Model  70  action. The bolt stop/release  202  has a front end  266  and a back end  268 . The bolt stop/release  202  may have a thin, flat body  270  that extends between the front and back ends  266 ,  268 . The bolt stop/release  202  may have an upright arm  272  which extends up from the body  270 . The arm  272  may jog to the left as it extends up from the body  270 , so that an upper portion  274  of the arm is offset, or spaced apart, to the left of the body. The front end of the body  270  is received within the left notch  162  of the base  102  of the first assembly  100  with sufficient clearance so that the bolt stop/release  202  does not push on the first assembly  100 . The upper portion  274  of the arm  272  extends up beside the left side  140 , outside the notch  162 . The left notch  162  may be sized and shaped to also receive the Winchester Model  70  ejector with clearance, or another left notch (not shown) may be provided for this purpose. 
     Referring to  FIG. 34-35 , side and bottom views show the first assembly  100  operatively arranged adjacent to a bolt stop/release  204  characteristic of a Remington  700  action. The bolt stop/release  204  has a front end  276  and a back end  278 . The bolt stop/release  204  may have a thin, flat body  280  that extends between the front and back ends  276 ,  278 . The bolt stop/release  204  may have an upper protrusion  282  in a rear portion of the body  280 . The front end of the body  280  is received within the left notch  162  of the base  102 . The protrusion  282  may also be received within the notch  162 . There is sufficient clearance so that the bolt stop/release  204  does not push on the first assembly  100 . The left notch  162  may be sized and shaped to also receive the Remington  700  ejector with clearance, or another left notch (not shown) may be provided for this purpose. 
       FIGS. 32-35  show two examples of bolt stop/releases  202 ,  204  designed for right-handed shooters. The first assembly  100  is also compatible with bolt stop/releases  202 ,  204  designed for left-handed shooters, in which case the bolt stop/releases are mirrored about a longitudinal vertical plane (such as plane  264 ) and received in the right notch  160  of the base  102 . Furthermore, various embodiments of the first assembly  100  may be provided with notches that are sized, shaped, and located to correspond to various firearm actions, such as a Borden action or a Defiance action. 
     Referring to  FIG. 36 , a second assembly  300  may include a plate sub-assembly  302 , an adapter block base sub-assembly  304 , and/or an adapter block  306 . Three adapter blocks  306 ,  308 ,  310  are shown for interchangeable connection to the adapter block base  304 . Each adapter block  306 ,  308 ,  310  is designed to securely couple to a particular style or class of receiver. These are but three examples. Preferably, each adapter block is designed so that the receiver may be secured to its adapter block using the receiver&#39;s front guard screw. The second assembly  300  may be referred to as a sighting holder or sighting assembly. The adapter block base sub-assembly  304  is shown operatively assembled to the plate sub-assembly  302 , which is operatively assembled to a rifle bench rest  400 . Referring briefly to  FIG. 45 , the plate sub-assembly  302  may be coupled to a main column  402  of the rifle bench rest  400 .  FIG. 36  also illustrates two more main columns  404 ,  406  for interchangeable assembly in the rifle bench rest  400 . The second assembly  300  has a front end  312 , a back end  314 , a right side  316 , and a left side  318 . 
     Referring to  FIG. 37 , the plate sub-assembly  302  may include a first plate  320 , second plate  322 , fasteners  324 ,  326 ,  328 ,  330 , spring plunger  332 , and pin  334 . The first plate  320  may be referred to as a lower plate and the second plate  322  may be referred to as a top plate. 
     The first plate  320  has five holes  350 ,  352 ,  354 ,  356 ,  358 . The holes  350 ,  352 ,  354 ,  356  extend through the first plate  320  along a top-bottom direction. The holes  350 ,  352 ,  354  are centrally located between the right and left sides  316 ,  318  in a linear arrangement from front to back. The holes  350 ,  352  are in the front half of the first plate  320  and the hole  354  is close to the back end  314 . The holes  350 ,  352  may have internal threads; or hole  352  may be unthreaded (smooth). The top end of the hole  352  may include a countersink.  FIG. 40  illustrates that the bottom end of the hole  352  may include a counterbore. The hole  354  may be circular, or elongated along a right-left direction, for example oval. The hole  356  is to the right rear of hole  354 . The hole  356  may be elongated along the right-left direction, or circular. The hole  358  extends through the first plate  320  along the right-left direction and intersects the hole  354 . The right and/or left portions of hole  358  may have internal threads. The first plate  320  may include an optional bullseye level or tube level (not shown) for preliminary leveling. 
     The second plate  322  has six holes  360 ,  362 ,  364 ,  366 ,  368 ,  370  that extend through the second plate along a top-bottom direction. The holes  360 ,  362 ,  364 ,  366 ,  368  are centrally located between the right and left sides  316 ,  318  in a linear arrangement from front to back. The hole  360  is near the front end  312 , corresponding to the location of hole  350  of the first plate  320 . The holes  362 ,  364 ,  366  are grouped in a central region of the second plate  322 . The hole  364  may be internally threaded. The hole  368  is near the back end  314 , corresponding to the location of hole  354  of the first plate  320 . The hole  370  is to the right rear of hole  368 , corresponding to the location of hole  356  of the first plate  320 . The hole  370  may be internally threaded. The second plate  322  may include an optional hole (not shown) corresponding to the location of hole  352  of the first plate  320 , to provide access for a driver to engage the head of the fastener  324  in hole  352  when the plate sub-assembly  302  is operatively assembled. The second plate  322  may include an optional bullseye level or tube level (not shown) for preliminary leveling. 
     The fastener  324  may be a countersunk head screw. 
     The fastener  326  may be referred to as a windage adjustment screw. The head of fastener  326  may be adapted for manual tightening and loosening. 
     The fastener  328  may be referred to as a windage lock screw. The head of fastener  328  may be adapted for manual tightening and loosening. 
     The fastener  330  may be a socket head shoulder bolt with a threaded tip  372  and a smooth shaft  374  between the tip  372  and the head. The fastener  330  may be referred to as a windage pivot bolt, whose central longitudinal axis may be referred to as a pivot axis. 
     The spring plunger  332  may have a cylindrical body  376  and a spring-loaded pin tip  378 . The body  376  may be smooth or externally threaded. 
     The plate sub-assembly  302  may be operatively assembled by inserting the fastener  324  through the hole  352  of the first plate  320  from top to bottom with the countersunk head in the countersink and the screw tip protruding from the bottom side of the first plate; positioning the second plate  322  on top of the first plate  320  with the front ends  312  facing the same direction, the right sides  316  facing the same direction, and the top sides facing the same direction; inserting the fastener  330  through the hole  360  from top to bottom and threading the tip  372  into the hole  350 ; press-fitting the top end of the pin  334  in the hole  368  so that the bottom end of the pin  334  protrudes from the bottom side of the second plate  322  and into the hole  354 ; inserting the fastener  328  through the hole  356  from bottom to top and threading the tip into the hole  370 ; inserting the spring plunger  332  into the left portion of the hole  358  so that the tip  378  is against the left side of the pin  334 ; and threading the fastener  326  into the right portion of the hole  358  so that the tip is against the right side of the pin  334 . 
     When the plate sub-assembly  302  is operatively assembled, clockwise and counterclockwise rotation of the fastener  326  causes the second plate  322  to rotate, or pivot, relative to the first plate  320  about the fastener  330 , against the resistance provided by the spring plunger  332 ; and clockwise and counterclockwise rotation of the fastener  328  locks and unlocks the rotation of the second plate relative to the first plate. Referring to  FIG. 38 , the fastener  326 , pin  334 , and spring plunger  332  function together as a windage fine adjustment mechanism.  FIG. 38  illustrates the radius of the arc along which the pin  334  travels as the second plate  322  rotates relative to the first plate  320 . The bottom end of the pin  334  may be received in the hole  354  with clearance, which may be provided all around or only along the right-left direction. The plate sub-assembly  302  may include indicia (not shown) to indicate the magnitude of adjustment right or left of a neutral (zero) position. The fastener  328  and pin  334  are centered in the holes  356 ,  354 , respectively, when the plate sub-assembly is in the neutral position. The neutral position is illustrated in  FIG. 36 . The fastener  328  in hole  356  and/or the pin  334  in hole  354  limit the range of motion or magnitude of windage adjustment provided by the plate sub-assembly  302 . 
     The adapter block base sub-assembly  304  may include a body  340 , fasteners  342 ,  344 , and pins  346 ,  348 . 
     The body  340  includes an undercut channel  380  which extends across the top side between the front and back ends  312 ,  314 . The undercut channel  380  may be a dovetail slot, T-slot, or other undercut geometry. The undercut channel  380  may be open (extend through) the front and/or back ends  312 ,  314 . Three holes  382 ,  384 ,  386  extend through the body  340  along a top-bottom direction, centrally located in the right-left width of the undercut channel  380 , in a linear arrangement from front to back. The hole  382  is near the front end  312 , corresponding to the location of hole  362  of the second plate  322 . The hole  384  is near the middle, corresponding to the location of hole  364  of the second plate. The hole  386  is near the back end  314 , corresponding to the location of hole  366  of the second plate. The hole  384  may be internally threaded or non-threaded (smooth). The top end of the hole  384  may include a counterbore ( FIG. 40 ). A hole  388  extends through the body  340  along a right-left direction and intersects the undercut channel  380  between the holes  384 ,  386 . The right and/or left portions of hole  388  may be internally threaded. 
     The fastener  342  may be referred to as a cross screw. The head of the fastener  342  may be adapted for manual tightening or loosening. 
     The fastener  344  may be a socket head cap screw. 
     The adapter block base sub-assembly  304  may be operatively assembled by fixing the pin  346  in the hole  382  so that the bottom end of the pin  346  protrudes below the bottom side of the body  340  and the top end of the pin  346  protrudes up into the undercut channel  380  (see  FIGS. 36, 39, and 40 ); inserting the fastener  344  into the hole  384  from top to bottom so that the head is received in the counterbore and the tip protrudes below the bottom side of the body  340 ; fixing the pin  348  in the hole  386  so that the bottom end of the pin  348  protrudes below the bottom side of the body  340  and the top end of the pin  348  is even with or recessed below the undercut channel  380  (see  FIG. 40 ); and inserting the fastener  342  into the hole  388  so that the tip protrudes from the opposite side of the body  340  from the head (see  FIGS. 38 and 39 ). The fastener  342  is illustrated with the head against the right side  316  and the tip protruding from the left side  318 . 
     When the adapter block base sub-assembly  304  is operatively assembled, the fastener  342  may be inserted into and removed from the hole  388  by a user. One or more of the fasteners  342 ,  344  and pins  346 ,  348  may be captive to the body  340 . 
     The plate sub-assembly  302  and the adapter block base sub-assembly  304  may be operatively assembled by inserting the bottom end of pin  346  into the hole  362 ; inserting the tip of the fastener  344  into the hole  364 , for example by threading the fastener  344  into the hole  364 ; and inserting the bottom end of pin  348  into the hole  366 . The pin  346  and holes  382 ,  362  may be a different diameter than the pin  348  and holes  386 ,  366 ; or the bottom end of pin  346  may protrude below the bottom side of the body  340  a different distance than the bottom end of pin  348 ; so that the plate sub-assembly  302  and the adapter block base sub-assembly  304  may only be assembled in a single orientation with the front ends  312  facing the same direction, the right sides  316  facing the same direction, and the top sides facing the same direction. 
     When the plate sub-assembly  302  and the adapter block base sub-assembly  304  are operatively assembled, the second plate  322  and the body  340  are rigidly secured together and mutually aligned along a front-back line extending through the holes  362 ,  364 ,  366 ,  382 ,  384 ,  386 . 
     Returning to  FIG. 36 , the adapter block  306  may be adapted to couple to the illustrated receiver  200 . The adapter block  306  includes an undercut rail  390  which extends across the bottom side between the front and back ends  312 ,  314 . The undercut rail  390  may be a dovetail rail, T-rail, or other undercut geometry, and is preferably complementary to the undercut channel  380  of the body  340  of the adapter block base sub-assembly  304 . The undercut rail  390  may extend across the entire bottom side, or a portion. A hole  392  may extend through the adapter block  306  along a top-bottom direction and may be located in a front half of the adapter block  306 . The hole  392  may be internally threaded to correspond to the external threads of the front guard screw for the receiver  200 . A notch  394  may extend across the top back end of the adapter block  306  along a right-left direction to form a step down from the top side. The notch  394  may have a  90  degree internal corner. 
     The adapter block assembly  308  may be adapted to couple directly to a scope  500 , without the first assembly  100  or receiver  200 . Preferably, the adapter block assembly  308  may couple to a 1″, 30 mm, or 34 mm scope. The adapter block assembly  308  may include a bottom block  702  and a top block  704 . Optional fasteners (not shown) may be included with the adapter block assembly  308 . This enables a shooter to test whether the scope  500  by itself has accurate come up adjustment or elevation adjustment, which may be of interest in military or other specialized shooting situations. 
     The bottom block  702  includes an undercut rail  706  which extends across the bottom side between the front and back ends  312 ,  314 . The undercut rail  706  may be a dovetail rail, T-rail, or other undercut geometry, and is preferably complementary to the undercut channel  380  of the body  340  of the adapter block base sub-assembly  304 . The undercut rail  706  may extend across the entire bottom side, or a portion. A V-groove  708  extends across the top side between the front and back ends  312 ,  314 . A tab  710 , or ear, may protrude from the top right side of the bottom block  702 ; a mirror image tab  712  may protrude from the top left side of the bottom block  702  ( FIG. 49 ). A hole  714  may extend through the tab  710  along a top-bottom direction and may include internal threads; an identical hole (not visible) may extend through the tab  712 . 
     The top block  704  includes a V-groove  718  that extends across the bottom side between the front and back ends  312 ,  314 . A tab  720 , or ear, may protrude from the bottom right side of the top block  704 ; a mirror image tab  722  may protrude from the bottom left side of the top block  704  ( FIG. 49 ). A hole  724  may extend through the tab  720  along a top-bottom direction and may include internal threads or may be smooth; an identical hole  726  ( FIG. 49 ) may extend through the tab  722 . 
     The adapter block assembly  308  may be operatively assembled by orienting the bottom and top blocks  702 ,  704  with the front ends  312  facing the same direction, the right sides  316  facing the same direction, and the top sides facing the same direction, so that the V-grooves  708 ,  718  face each other, the tabs  710 ,  720  face each other, and the tabs  712 ,  722  face each other. Fasteners (not shown), such as screws, may be inserted through the holes in the tabs to lock the bottom and top blocks  702 ,  704  together. 
     The adapter block  310  may be adapted to couple to a standard Remington  700  or round clone receiver. The round receiver is held rigidly in the V block  310  so that the action will not roll right or left as the scope is being leveled, as discussed below. The adapter block  310  includes an undercut rail  728  which extends across the bottom side between the front and back ends  312 ,  314 . The undercut rail  728  may be a dovetail rail, T-rail, or other undercut geometry, and is preferably complementary to the undercut channel  380  of the body  340  of the adapter block base sub-assembly  304 . The undercut rail  728  may extend across the entire bottom side, or a portion. A V-groove  730  extends across the top side between the front and back ends  312 ,  314 . A hole  732  may extend through the adapter block  310  along a top-bottom direction and may be located in a front half of the adapter block  310 . The hole  732  may be internally threaded to correspond to the external threads of the front guard screw for the standard Remington  700  or round clone receiver. Other action blocks are contemplated for various actions, each including a hole to receive the corresponding front guard screw. Referring to  FIG. 40 , a notch  734  may extend into the bottom front end of the adapter block  310 . The notch  734  may be sized, shaped, and located to receive the top end of the pin  346 . A transverse groove  736  may extend across the bottom side of the adapter block  310  between the right and left sides  316 ,  318  in the rear half of the adapter block. The groove  736  may be sized, shaped, and located to receive a portion of the fastener  342 . 
     Referring to  FIGS. 39-40 , each adapter block may be interchangeably operatively assembled to the adapter block base sub-assembly  304 . The adapter block  310  is shown as an example. The adapter block  310  may be operatively assembled to the adapter block base sub-assembly  304  by sliding the front end of the undercut rail  728  into the back end of the undercut channel  380  of the body  340  of the adapter block base sub-assembly  304  until the top end of the pin  346  enters the notch  734  and inserting the fastener  342  through the hole  388  and the groove  736 . 
     When the adapter block  310  is operatively assembled to the adapter block base sub-assembly  304 , the adapter block  310  is rigidly secured to the adapter block base sub-assembly and aligned along a front-back line extending through the holes  382 ,  384 ,  386 . 
     Returning to  FIG. 36 , the rifle bench rest  400  may be a prior art apparatus such as those marketed by Sinclair, Hart, and Wichita. Not every part or feature of the rifle bench rest  400  will be described. The rifle bench rest  400  may include a main column  402  ( FIG. 45 ) and three adjustable feet  410 ,  412 ,  414  ( FIG. 51 ). The main column  402  may be replaced by the main column  404  or the main column  406 . The top end of each main column  402 ,  404 ,  406  may include an internally threaded hole  422  (not visible),  424 ,  426 . Each foot  410 ,  412 ,  414  is adjustable up and down to raise or lower the foot as needed to make the main column precisely vertical or achieve other alignment goals. 
     Referring to  FIG. 41 , the adapter block  306 , adapter block base sub-assembly  304 , plate sub-assembly  302 , and rifle bench rest  400  are shown operatively assembled together. 
     Referring to  FIGS. 42 and 43 , the receiver  200  may be operatively assembled to the adapter block  306  by placing the bottom surface  216  of the receiver  200  against the top side of the adapter block  306  with the front ends  210 ,  312  facing the same way and inserting the front guard screw for the receiver  200  through the hole  392  and into the hole  218 . The receiver  200  and adapter block  306  may then be operatively assembled to the adapter block base sub-assembly  304 . The adapter block base sub-assembly  304 , plate sub-assembly  302 , and rifle bench rest  400  may already be operatively assembled together.  FIG. 43  shows that the fastener  330  is preferably centered under the receiver axis  214  so that the axis  214  intersects the pivot axis of the fastener  330 .  FIG. 45  shows that the foot  410  of the rifle bench rest  400  is preferably centered under the receiver axis  214 . The first assembly  100  may be operatively assembled to the receiver  200  at any step in this procedure, preferably after the receiver  200 , adapter block  306 , adapter block base sub-assembly  304 , plate sub-assembly  302 , and rifle bench rest  400  have been assembled.  FIG. 43  shows a barrel, in dashed lines, coupled to the receiver  200 . While not shown for clarity, the trigger assembly, ejector, and/or other assemblies and parts normally coupled to the receiver  200  may remain attached. However, preferably, the magazine well assembly and stock are removed. 
     Similarly, the standard Remington  700  or round clone receiver may be operatively assembled to the adapter block  310  by placing the bottom side of the action in the V-groove  730  with the front ends facing the same way and inserting the front guard screw for the action through the hole  732  and into the front guard screw hole of the action. 
     Referring to  FIG. 44 , after the first assembly  100 , receiver  200 , adapter block  306 , adapter block base sub-assembly  304 , plate sub-assembly  302 , and rifle bench rest  400  have been assembled, the combined apparatus may be positioned with the receiver  200  visually aimed downrange. The feet  410 ,  412 ,  414  of the rifle bench rest  400  may be adjusted up or down until the bubble in the tube level  106  of the first assembly  100  is centered. The feet  410 ,  412 ,  414  may also be adjusted until the bubble in the bullseye level  104  is centered. When the bubbles in the tube level  106  and/or the bullseye level  104  are precisely centered, the receiver  200  and its axis  214  are precisely leveled with respect to Earth&#39;s gravity. Preferably, the feet  410 ,  412 ,  414  are adjusted until the bubbles in the tube level  106  and the bullseye level  104  are simultaneously precisely centered. 
     Referring to  FIG. 45 , a scope  500  may be coupled to the precisely leveled receiver  200 . The scope  500  is shown schematically in this figure. The scope  500  may be coupled to the receiver  200  before the receiver has been precisely leveled. Either way, at this stage, the scope  500  is preferably coupled to the receiver  200  loosely enough that the scope may be reoriented relative to the receiver. A target  600  may be mounted to a target frame (not shown) and set up downrange, for example, at  100  yards,  200  yards,  500  yards,  1000  yards, etc. Preferably, the target  600  may be mounted to the frame with reference to a plumb line or other true vertical datum (not shown) so that vertical indicia  602  on the target  600  are precisely vertical. A plumb line may be used instead of the target  600 . 
     Elevation at the downrange target  600  may be adjusted at foot  410 , which is preferably in the rear and centered under the central longitudinal axis  214  of the receiver  200 . As the vertical reticle image can span about 48″ at 100 yards or meters, and the height of the target image downrange can also vary, the rear post adjustment screw (foot  410 ) is a practical means for elevation adjustment. Elevation may be adjusted while viewing the target  600  through the scope  500 . Preferably, elevation is adjusted so that the horizontal reticle is even with the center point of the target. 
     Windage at the target  600  may be adjusted using the plate sub-assembly  302 , preferably without repositioning the rifle bench rest  400 , preferably without repositioning the feet  410 ,  412 ,  414 . Fastener  326  may be turned clockwise and counterclockwise to adjust windage while viewing the target  600  through the scope  500 . Preferably, windage is adjusted so that the vertical reticle is even with the center point of the target. At this stage, the vertical and horizontal reticles may not be truly vertical and horizontal, but their intersection is superimposed over the o center point of the target. Fastener  328  may be tightened to lock the plate sub-assembly  302  when windage is precisely adjusted, or loosened to permit further windage adjustment. The illustrated plate sub-assembly  302  provides over 18″ of right to left horizontal adjustment or correction at 100 yards or meters. 
     The scope  500  may be rotated right or left to align its vertical reticle to a vertical datum at the target  600 , such as a plumb line, a 4 foot sheet rock level, a commercial target, a laser generated line, or a target having two closely-spaced vertical indicia  602 . The scope  500  may be rotated while viewing the target  600  through the scope. Preferably, the receiver  200  remains precisely leveled while the scope  500  is rotated. After the vertical reticle is precisely aligned to the vertical datum at the target  600 , the scope  500  may be tightly secured to the receiver  200 . This may involve multiple iterations of incrementally tightening the scope fittings, checking the alignment of the vertical reticle by viewing the vertical datum through the scope, checking the levels  104 ,  106 , and rotating the scope to realign the vertical reticle to the vertical datum. 
     While the preceding steps of elevation adjustment, windage adjustment, and scope rotation are described in an order, the steps may be performed in any order and any step may be performed more than once during scope alignment. 
     After the scope has been aligned and tightly secured to the receiver  200 , a scope level (not shown) may be attached, aligned so that its bubble is centered, and fixed to the aligned scope  500 . Performing this step at this stage may ensure that the scope level is actually “true,” i.e., the scope level bubble is centered when the scope  500  and receiver  200  are truly level. The scope level may then be relied upon in the field when aiming in on a target. 
     Referring to  FIGS. 46-48 , the adapter block  310 , adapter block base sub-assembly  304 , plate sub-assembly  302 , and rifle bench rest  400  are shown operatively assembled together. 
     Referring to  FIG. 49 , the adapter block assembly  308 , adapter block base sub-assembly  304 , plate sub-assembly  302 , and rifle bench rest  400  are shown operatively assembled together. 
     Referring to  FIGS. 50-52 , the scope  500  is shown operatively assembled in the adapter block assembly  308 . This arrangement enables a scope to be evaluated for accurate movement in response to scope turret actuation. In other words, this arrangement tests a scope for accurate elevation (come up) and/or windage adjustment. 
     Referring to  FIG. 53 , a bottom side of an AR action  750  is shown. The AR action includes a receiver  752  having a planar bottom surface  754 , a front takedown boss  756 , a rear takedown boss  758 , and a chamber  764 . The bottom surface  754 , particularly between the front and rear takedown bosses  756 ,  758 , is a reliable precision machined planar surface which is an excellent primary datum feature for the disclosed technology. The bottom surface  754  between the front and rear takedown bosses  756 ,  758  may be referred to as a planar datum surface. The front takedown boss  756  includes a front takedown hole  760  and the rear takedown boss  758  includes a rear takedown hole  762 . The chamber  764  is a cavity which extends into the receiver  752  from the bottom side. The AR action also includes a rear takedown pin  766 , visible in  FIG. 54 . Many variants now exist based upon the original ArmaLite design: AR-10, AR-15, M1, M4, M16, etc. In 2009, the term “modern sporting rifle” was coined by the National Shooting Sports Foundation to describe modular semi-automatic rifles including AR-15s. Today, nearly every major firearm manufacturer produces its own generic AR-15 style rifle. 
     Referring to  FIGS. 54-61 , the AR action  750  is shown coupled to a scope  770 . A third assembly  800  may be an adaptation of the principles disclosed above for the first and second assemblies  100 ,  300 , specific to the AR action  750 . The third assembly  800  may thus be referred to as an AR sighting holder or AR sighting assembly. The third assembly  800  may include an adapter block  802  which functions like the adapter blocks  306 ,  308 ,  310  to securely couple to the AR receiver  752 , and a plate sub-assembly  804  which functions like the plate sub-assembly  302 . The third assembly  800  is shown coupled to the rifle bench rest  400 . 
     The adapter block  802  functions like the adapter blocks  306 ,  308 ,  310  to securely couple the AR receiver  752  to the plate sub-assembly  804 . The adapter block  802  includes front, rear, right, left, top, and bottom sides  810 ,  812 ,  814 ,  816 ,  818 ,  820 . A transverse notch  822  may extend from right to left across the front side  810  and the top side  818  to provide clearance for the front takedown boss  756  of the receiver  752  when the bottom surface  754  is against the top side  818 . A boss  824  may extend upwardly from the top side  818 . The boss  824  may be sized, shaped, and located to be received within the chamber  764  of the receiver  752 , preferably within a front region of the chamber  764  near the front takedown boss  756 . A substantially rectangular boss  824  is illustrated. The boss  756  may fit within the chamber  764  with generous front and back clearance and little to no right and left clearance. A slot  826  may extend into the top side  818 , and may be sized, shaped, and located to receive the rear takedown boss  758  of the receiver  752  when the bottom surface  754  is against the top side  818 . The slot  826  may include generous front and back clearance and little to no right and left clearance relative to the rear takedown boss  758 . A transverse hole  828  may extend from right to left through the adapter block  802  and crossing the slot  826 . The hole  828  may be sized, shaped, and located to receive the rear takedown pin  766  when the bottom surface  754  is against the top side  818  and the rear takedown boss  758  is in the slot  826 . Additional transverse holes  830 ,  832  are shown in front of and in back of the hole  828 ; the additional holes  830 ,  832  may be provided to accommodate AR receivers with different rear takedown boss locations. A group of three holes  834 ,  836 ,  838  may extend from top to bottom through the adapter block  802 , which may function like the group of holes  382 ,  384 ,  386  of the body  340  of the adapter block base sub-assembly  304  to receive fastener  344  and pins  346 ,  348  to couple the adapter block  802  to the plate sub-assembly  804 . The holes  834 ,  836 ,  838  are shown extending through the boss  824  in this example. A hole  840  may extend from top to bottom through the adapter block  802 , to the rear of hole  838 . The hole  840  may receive a fastener to further couple the adapter block  802  to the plate sub-assembly  804 . Referring to  FIG. 56 , the adapter block  802  may include the bullseye level  104  secured by the screws  114 ,  116 ,  118  and the tube level  106  secured by the set screws  120 ,  122 , as described for the first assembly  100 . 
     The plate sub-assembly  804  functions like the plate sub-assembly  302 , including coupling to the main column  402  of the rifle bench rest  400 , coupling to the adapter block  802 , and providing a windage fine adjustment mechanism. The plate sub-assembly  804  may include a first plate  850  and a second plate  852 , and may use fasteners  324 ,  326 ,  328 ,  330 , spring plunger  332 , and pin  334 . The first plate  850  may be referred to as a lower plate and the second plate  852  may be referred to as a top plate. The first plate  850  may couple to the rifle bench rest  400 . The second plate  852  may couple to the adapter block  802 . The first and/or second plate  850 ,  852  may include an optional bullseye level or tube level (not shown) for preliminary leveling. The plate sub-assembly  804  may be operatively assembled by a method similar to that described above for the plate sub-assembly  302 . 
     Referring to  FIGS. 54-55 , the receiver  752  may be operatively assembled to the adapter block  802  by orienting the receiver and adapter block with the front ends facing the same way, and the bottom surface  754  of the receiver facing the top side  818  of the adapter block; inserting the rear takedown boss  758  into the slot  826 ; aligning the rear takedown hole  762  with the hole  828 ; inserting the rear takedown pin  766  through the holes  762 ,  828 ; inserting the boss  824  into the chamber  764 ; receiving the front takedown boss  756  in the notch  822 ; and placing the bottom surface  754  against the top side  818 . The steps of inserting the rear takedown boss  758  into the slot  826 , aligning the rear takedown hole  762  with the hole  828 , and inserting the rear takedown pin  766  through the holes  762 ,  828  may be performed with the front end of the receiver  752  tilted up as shown. The nominal dimensions and tolerances between the receiver  752  and adapter block  802  may be selected so that when the receiver is assembled to the adapter block, the bottom surface  754  is in surface contact with the top side  818 , in other words, has at least three points of mutual contact. Thus, the top side  818  may function as a primary planar datum surface of the adapter block  802 . While this description is made in the context of a continuous flat planar top side  818 , it is contemplated that the top side  818  may instead comprise discontinuous patches, or points, for contacting the bottom surface  754 . Taken together, the discontinuous patches or points may function as a primary planar datum surface. Whether the top side  818  is continuous or discontinuous, the top side  818  preferably contacts the full length of the bottom surface  754  between the between the front and rear takedown bosses  756 ,  758 . 
     The adapter block  802  may be operatively assembled to the plate sub-assembly  804  by a method similar to that described for the adapter block  310 , the adapter block base sub-assembly  304 , and the plate sub-assembly  302 , modified of course due to the adapter block  802  incorporating features of the adapter block base sub-assembly  304 . 
     The methods of using the third assembly  800  to first precisely level the AR receiver  752  and then precisely level the reticles of the scope  770  are according to the principles laid out above for the first and second assemblies  100 ,  300 . 
     All parts of the apparatus described herein are preferably fabricated from substantially rigid materials, for example metals or ceramics, except for parts that are described as compliant, elastic, deformable, springs, and the like. 
     Any methods disclosed herein includes one or more steps or actions for performing the described method. The method steps and/or actions may be interchanged with one another. In other words, unless a specific order of steps or actions is required for proper operation of the embodiment, the order and/or use of specific steps and/or actions may be modified. 
     Reference throughout this specification to “an embodiment” or “the embodiment” means that a particular feature, structure or characteristic described in connection with that embodiment is included in at least one embodiment. Thus, the quoted phrases, or variations thereof, as recited throughout this specification are not necessarily all referring to the same embodiment. 
     Similarly, it should be appreciated that in the above description of embodiments, various features are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure. This method of disclosure, however, is not to be interpreted as reflecting an intention that any claim require more features than those expressly recited in that claim. Rather, as the following claims reflect, inventive aspects lie in a combination of fewer than all features of any single foregoing disclosed embodiment. Thus, the claims following this Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment. This disclosure includes all permutations of the independent claims with their dependent claims. 
     Recitation in the claims of the term “first” with respect to a feature or element does not necessarily imply the existence of a second or additional such feature or element. Elements recited in means-plus-function format are intended to be construed in accordance with 35 U.S.C. § 112 Para. 6. It will be apparent to those having skill in the art that changes may be made to the details of the above-described embodiments without departing from the underlying principles of the technology. 
     While specific embodiments and applications of the present technology have been illustrated and described, it is to be understood that the technology is not limited to the precise configuration and components disclosed herein. Various modifications, changes, and variations which will be apparent to those skilled in the art may be made in the arrangement, operation, and details of the methods and systems of the present technology disclosed herein without departing from the spirit and scope of the technology.