Abstract:
A sighting system for a fire arm, e.g. machine gun includes front and rear sight assemblies. The rear sight assembly has a pivotal L-shaped member having two different sized apertures. Pivotal and windage lateral movements of the L-shaped member are achieved via a threaded shaft and a knob held in place via a detent screw/plunger assembly. The L-shaped member is secured in place via the detent plunger spring assembly. The front sight assembly is configured for windage lateral movement and has a sight post that is raised and lowered and secured in place via detent plunger spring assemblies. Another embodiment includes a rear sight assembly which may be configured for windage lateral movement. Detent plunger spring assemblies secure an aperture member of the rear sight assembly and the front sight assembly during operation of the fire arm, and the front sight assembly is configured for elevational and windage lateral movements.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is based on U.S. Provisional Patent Application No. 60/964,248 filed Aug. 10, 2007, on which priority of this patent application is based and which provisional patent application is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a sighting system for a machine gun, such as an M2HB, M3 flexible, which is a 50 caliber machine gun. More specifically, the invention relates to a sighting system having a rear sight assembly which is adjustable for sighting in long-range and short-range targets and a front sight assembly. 
     2. Description of Related Art 
     Rear and front sights are presently used on machine guns for sighting long-range and short-range targets. In this respect, both the rear sight and the front sight are mounted on the receiver of the machine gun. The rear sight is mounted at the rear end of the machine gun and the front sight is mounted at the front of the machine gun, closer to the target. 
     One such arrangement involves a front sight that is in a fixed position. The front sight has an arc housing mounted on a base which is mounted on a trunnion of the receiver. The housing defines an aperture and includes a sight post extending upwardly in the front sight aperture. 
     The rear sight of this aforesaid arrangement is attached to the upper receiver. The upright bracket may be attached to a rod which is operatively connected to a screw for rotation of the rod, and therefore, lateral movement of the bracket within the trunnion. The upright bracket may have an elongated slot having a slidable aperture and calibration marks along one of its surfaces for positioning the aperture relative to a target. The aperture is attached to an elevational screw which slides on a rail for setting the elevation and a leaf assembly transverses the screw on the bottom. 
     A disadvantage of the sighting system of the aforesaid arrangement for a machine gun is that the upright bracket of the rear sight can easily become askew on its threaded rod when the machine gun is being fired such that the aperture also becomes askew, thereby disturbing the line-of-sight of the target. A further disadvantage is that the rear sight does not provide different apertures for varying distant targets. 
     Additional examples of rear and/or front sights for various types of fire arms are disclosed in U.S. Pat. Nos. 4,127,943; 4,536,966; 5,930,906; 5,983,774; 6,860,056 B2; 6,968,643 B2; and 7,181,882 B2. 
     U.S. Pat. No. 7,181,882 B2 discloses a sighting system for a shotgun having a rear sight and a front sight. The rear sight has sight elements adaptable for “zeroing in” the same gun with different ammunition. The rear sight has windage and elevation adjustments for the sight elements so that multiple ammunition types having different trajectories may be fired accurately from a single firearm after zeroing in one of the sight elements for each of the different ammunition types. The sight elements may be connected to each other or to a common pivot arm or movable bracket so that moving one sight element into the sight path automatically removes the other from the line of vision. In one embodiment, one of the elements of the rear sight is a hollow ghost ring used to align shots taken with buckshot-type ammunition and another element is a V-shaped notch used to align shots taken with slug loads. These elements are attached to a pivoting L-shaped mount and form a single pivoting unit and are separated by a right angle. Consequently, flipping the V-shaped notch into the line-of-sight pivots the ghost ring out of the sight plane and vice versa. Applying slight pressure to the pivoting system flips the sights and claps, or other mechanisms may be desirable to fix the sights in a given position. 
     This design for the sighting system for a shotgun as disclosed in the above U.S. Pat. No. 7,181,882 B2 may be adequate for a shotgun and the other sighting systems of the prior art listed hereinabove may be adequate for the type of firearm they are used on; however, these sighting systems prove to be ineffective when used on a machine gun. That is, the sighting system for use on a machine gun needs to have a greater degree of adjustability for setting up a line-of-sight for a target. Also, since a machine gun is designed for continuous use and is a powerful, heavy weapon weighing close to 100 pounds, it creates a great thrust when operated. Also, the sights, particularly the rear sight close to the operator of the machine gun, needs to remain in place when the machine gun is continuously being fired so that the line-of-sight remains accurate even though the target may be moving quickly. 
     There is a need, therefore, in the art pertaining to machine guns, such as an M2HB, M3 flexible, which are 50 caliber machine guns, for an improved design of a sighting system having a front sight assembly and a rear sight assembly which allows for an increase in the percentage of hit rounds when the machine gun is operated and which also allows for a greater degree of adjustability compared to the prior art sighting systems used on firearms, including those described hereinabove. 
     SUMMARY OF THE INVENTION 
     The present invention has met this need. The present invention provides a sighting system for a machine gun that includes an adjustable front sight assembly and an adjustable rear sight assembly, both mounted on the top surface of a receiver of a machine gun. In a first embodiment, the front sight assembly includes a first aperture member having a sight post, a first base member fixed to the first aperture member and having a dovetail surface, and a second base member or dovetail base having a dovetail surface corresponding to and slidable within the dovetail surface of the first base member. The second base member is mounted into slots on top of the surface of a trunnion which is the forward section of the machine gun, and the first aperture member via the dovetail base connection can be moved in a lateral direction perpendicular to the longitudinal plane of the top surface of the trunnion. The sight post has an upper needle portion which extends upwardly into the aperture defined by an upper arched portion of the aperture member and a lower elongated body member that extends downwardly into the lower portion of the first base member. The sight post is configured to be raised and lowered within the window of the first aperture member. This is accomplished by providing an external threaded portion on the elongated body of the sight post and by providing an annular elevation knob which encircles the elongated body of the sight post and which has a threaded portion on an inner annular surface that meshes with the external threaded portion on the elongated body of the sight post. Rotation of the elevation knob in one direction raises the sight post and rotation in an opposite direction lowers the sight post within the window of the first aperture member. 
     The rear sight assembly includes a base plate which is mounted along the longitudinal plane of the top rear receiver of the machine gun. Fastened to this elongated base plate are a base support cradle and a second aperture member which includes an L-shaped member, pivotally mounted in the base support cradle. A first leg of the L-shaped member has an upper circular portion with an aperture and a second leg of the L-shaped member has the shape of an inverted “V” with an aperture. The aperture of the first leg generally is greater than the aperture of the second leg. Pivoting the first leg having the larger aperture into the line-of-sight pivots the second leg having the smaller aperture out of the sight plane and vice versa. The L-shaped member is connected to an elongated shaft extending through a first longitudinal opening in the lower portion of the L-shaped member and through the support members of the base support cradle for the pivotal movement of the first leg and the second leg into and out of the sight plane. This pivotal movement of the L-shaped member, which generally is about 90 degrees, is done manually by the operator of the machine gun. The elongated shaft connecting the L-shaped member to the base support cradle has a threaded portion which corresponds to an internal threaded portion in the first longitudinal opening of the lower portion of the L-shaped member. This elongated shaft is operatively connected to an external knob which rotates the shaft and causes the L-shaped member to move laterally along the length of the shaft for lateral windage movement of the L-shaped member relative to the base support cradle and perpendicularly relative to the longitudinal plane of the top surface of the receiver of the machine gun. 
     The lower portion of the L-shaped member also has a second longitudinal opening located adjacent to the first longitudinal opening. A detent plunger spring assembly extends through this second longitudinal opening. This detent plunger spring assembly is spring biased and includes two opposed plunger members separated by a spring. The plunger members extend out of the second longitudinal opening of the lower portion of the L-shaped member on either side of the L-shaped member and are configured to engage into openings in the two sidewalls of the support members of the base support cradle. Each sidewall has two such openings which are located at a 90 degree angle relative to each other. Thus, when the L-shaped member is manually pivoted 90 degrees from a first position to a second position within the sidewalls of the base support cradle, the two plunger members are forced inwardly toward each other to become disengaged from a first set of openings, i.e., one opening on each of the two sidewalls of support members of the base support cradle, and then are forced outwardly to engage the second set of openings, i.e., one opening on each of the two sidewalls of the support members of the base support cradle, which are located at a 90 degree angle relative to the first set of openings. This detent plunger spring assembly registers and retains the L-shaped pivotal member in position within the base support cradle and still allows the L-shaped pivotal member to be moved laterally for windage purposes. A detent screw/plunger assembly located within the windage screw knob engages notches located in a bushing to prevent the rotation of the knob and therefore the elongated shaft. 
     A second embodiment of a sighting system of the invention includes a front sight assembly and a rear sight assembly. The rear sight assembly can be pivoted upwardly in an operative position or it can be pivoted downwardly longitudinally along the receiver supporting the machine gun. In both instances, an aperture member is fixed in position via a detent plunger spring arrangement which includes a first detent plunger spring assembly and a second detent plunger spring assembly, each located in a leg member of the L-shaped member of the aperture member. The detent plunger spring assemblies for this rear sighting assembly are similar to those used in the rear sight assembly of the first embodiment. In this second embodiment, the rear sight assembly may be non-movable or it may be movable. In the latter instance, the aperture member is configured for windage lateral movement along a shaft extending through the aperture member, and a windage screw assembly including a knob is attached to an end of the shaft. Rotation of the knob forces the aperture member to move along a threaded portion of the shaft and moves the aperture member to the right and the left of a base support cradle which supports the aperture member. 
     The front sight assembly is configured for windage lateral movement and elevational movement. The windage lateral movement is accomplished via a windage screw assembly similar to that for the rear sight assembly for laterally moving an aperture member of the front sight assembly and the aperture member, and its sight post is retained with its base support cradle and fixed in this position via a detent plunger spring arrangement having two detent plunger spring assemblies similar to the aperture member of the rear sight assembly. The aperture member is pivotally connected to the base support cradle and is configured to be pivoted at a 90 degree angle for stowing purposes and in an opposite pivotal position for operation of the machine gun. The aperture member includes a threaded elongated shaft and a locking nut knob is mounted around the threaded elongated shaft. The locking nut knob locks the aperture in a fixed position. Elevation of the aperture is achieved by unlocking the nut knob and rotating the aperture in a clockwise or counterclockwise direction. The aperture member and its sight post are held in a desired elevational position via V-shaped head plunger assemblies located diametrically opposite each other relative to the length of the threaded elongated shaft of the aperture member. The V-shaped head of these plunger assemblies engage in an elongated V-groove in the threaded elongated shaft of the aperture member to lock the aperture member and its sight post in a desired elevational position. 
     It is therefore an object of the invention to provide an improved design for a rear sight assembly and a front sight assembly of a sighting system for a powerful firearm such as a machine gun, e.g., an M2HB, M3 flexible which are 50 caliber machine guns, weighing close to 100 pounds, wherein the sight line remains fixed and on target regardless of the speed of the target or the firing frequency of the machine gun. More specifically, in the invention, the number of rounds that can be hit are increased from about 10% to about 90% compared to the machine guns of the prior art using prior art sighting systems. 
     It is a further object of the invention to provide a heavy-duty mechanical rear sight assembly and a heavy-duty mechanical front sight assembly for a powerful firearm, e.g., a machine gun, wherein the front sight assembly can be moved laterally for windage, and the sight post can be adjusted to be raised and lowered for elevation, and wherein the rear sight assembly has a fold-down member or a pivotal member that provides different-sized apertures corresponding to the varying distances of the target. The pivotal member can be moved laterally for windage purposes and can be fixed in place during continuous operation of the firearm. 
     It is still a further object of the invention to provide a powerful firearm such as a machine gun, with an adequate rear sight assembly including a pivotal member. The pivotal member provides at least two different sized apertures corresponding to the different distances of the target and the different trajectories of the ammunition and at least one detent plunger spring assembly on at least the rear sight assembly. The detent plunger spring assembly registers and retains the pivotal member into position when the pivotal member is forced from a first position to a second position and still allows lateral movement of the pivotal member for windage purposes. The detent plunger spring assembly also fixes the pivotal member in position during continuous operation of the firearm. 
     A further object of the invention is to provide a sighting system for a machine gun which provides a rear sight assembly and a front sight assembly, both of which have a greater degree of adjustability for accommodating the varying distances of the target and the varying trajectories of the ammunition compared to the sighting system of the prior art for machine guns. The apertures of the rear sight assembly and the front sight assembly are spring biased into position and remain in a fixed position regardless of the frequency and number of times the machine gun is operated. 
     These and other objects and advantages of the invention will be made apparent from the following description taken together with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view of a movable rear sight assembly of a first embodiment of the invention illustrating a first leg of an L-shaped pivotal member of an aperture member and shown, in phantom, the internal structure and some components of the rear sight assembly; 
         FIG. 2  is a front view of the rear sight assembly of  FIG. 1  illustrating a second leg or the rear peep sight leg of the L-shaped pivotal member of the aperture member and shown, in phantom, the internal structure and some components of the rear sight assembly; 
         FIG. 3  is a top view of the rear sight assembly of  FIG. 1  showing the L-shaped pivotal member of the aperture member in its positioning of  FIG. 1 ; 
         FIG. 4  is a left-side elevation view of the rear sight assembly of  FIG. 1  showing the L-shaped pivotal member of the aperture member in its positioning of  FIG. 3 ; 
         FIG. 5  is a smaller scale left-side elevation view of the rear sight assembly of  FIG. 1  showing the L-shaped member of  FIG. 4  pivoted 90 degrees to the left; 
         FIG. 6   a  is a cross-sectional view of the rear sight assembly taken along lines  6   a - 6   a  of  FIG. 3 ; 
         FIG. 6   b  is a front view of a detent plunger spring assembly for the aperture member of  FIG. 6   a  which may be used to repair the detent plunger spring assembly of  FIG. 1 ; 
         FIG. 7   a  is a front view of the aperture member of  FIG. 1  showing longitudinal openings in phantom; 
         FIG. 7   b  is a side view of the aperture member of  FIG. 7   a;    
         FIG. 7   c  is a partial front view of the aperture member of  FIG. 7   a  showing in cross section the longitudinal openings and taken along lines  7   c - 7   c  of  FIG. 7   b;    
         FIG. 8   a  is a front view of a detent plunger spring assembly which may be used as an alternate or replacement for the detent plunger spring assemblies for the aperture member of  FIG. 6   a  and showing in phantom its components; 
         FIG. 8   b  is a front view of a guide rod of the detent plunger spring assembly of  FIG. 8   a;    
         FIG. 9  is a front view of a front sight assembly for use with the rear sight assembly of  FIG. 1  and showing in phantom its internal structure and containing enlarged views A and B; 
         FIG. 10  is a rear view of the front sight assembly of  FIG. 9 ; 
         FIG. 11  is a side view of the front sight assembly of  FIG. 9 ; 
         FIG. 12  is a cross-sectional side view of the front sight assembly of  FIG. 11 ; 
         FIG. 13   a  is a rear view of a fixed rear sight assembly of a second embodiment of the invention and showing in phantom the internal structure and some components of the rear sight assembly; 
         FIG. 13   b  is a front view of a detent plunger spring assembly for use in the fixed rear sight assembly of  FIG. 13   a  which may be used to repair the detent plunger spring assemblies of  FIG. 13   a;    
         FIG. 14  is a top view of the rear sight assembly of  FIG. 13   a  and showing in phantom the internal structure and some components of the rear sight assembly; 
         FIG. 15  is a right-side view of the rear sight assembly of  FIG. 13   a  showing an L-shaped pivotal member of an aperture member in its positioning of  FIGS. 13   a  and  14 ; 
         FIG. 16  is a smaller scale right-side view of the rear sight assembly showing the L-shaped pivotal member of the aperture member of  FIG. 15  and its positioning when pivoted 90 degrees to the right as indicated by the arrow in  FIG. 15 ; 
         FIG. 17  is a cross-sectional view of the rear sight assembly taken along lines  17 - 17  of  FIG. 14 ; 
         FIG. 18   a  is a front view of a detent plunger spring assembly which may be used as an alternate or replacement for the detent plunger spring assemblies for the aperture member of  FIG. 13   a  and showing in phantom its components; 
         FIG. 18   b  is a front view of a guide rod of the detent plunger spring assembly of  FIG. 18   a;    
         FIG. 19   a  is a rear view of a movable rear sight assembly of a second embodiment of the invention and showing in phantom the internal structure and some components of the rear sight assembly; 
         FIG. 19   b  is a front view of a detent plunger spring assembly for use in the movable rear sight assembly of  FIG. 19   a  which may be used to repair the detent plunger spring assemblies in  FIG. 19   a;    
         FIG. 20  is a top view of the movable rear sight assembly of  FIG. 19   a;    
         FIG. 21  is a smaller scale right-side view of the movable rear sight assembly of  FIG. 19   a  and showing an L-shaped pivotal member of an aperture member in its positioning of  FIGS. 19   a  and  20 ; 
         FIG. 22  is a right-side view of the movable rear sight assembly of  FIG. 19   a  and showing the L-shaped pivotal member pivoted 90 degrees to the right as shown in  FIG. 21 ; 
         FIG. 23  is a cross-sectional view of the L-shaped pivotal member of an aperture member taken along lines  23 - 23  of  FIG. 20 ; 
         FIG. 24   a  is a front view of a detent plunger spring assembly which may be used as an alternate or replacement for the detent plunger spring assemblies for the aperture member of  FIG. 19   a  and showing in phantom its components; 
         FIG. 24   b  is a front view of a guide rod of the detent plunger spring assembly of  FIG. 24   a;    
         FIG. 25  is a rear view of a front sight assembly for use with the rear sight assembly of  FIG. 13   a  and  FIG. 19   a  and showing in phantom its internal structure and some of its components and containing an enlarged view A; 
         FIG. 26  is a full rear view of the front sight assembly of  FIG. 25  and includes a trunnion adapter for attaching the front sight assembly to a receiver of a machine gun; 
         FIG. 27  is a right-side view of the front sight assembly of  FIG. 25 ; 
         FIG. 28  is a full right-side view of the front sight assembly of  FIG. 26 ; 
         FIG. 29  is a full right-side view of the front sight assembly of  FIG. 28  and showing the aperture arrangement in a stowing position as indicated by the arrow in  FIG. 28 ; 
         FIG. 30   a  is a full front view of the front sight assembly of  FIG. 26 ; 
         FIG. 30   b  is a front view of a detent plunger spring assembly for use in the movable front sight assembly of  FIGS. 25-30   a  which may be used to repair the detent plunger spring assemblies of  FIGS. 25-30   a;    
         FIG. 31  is a top plan view illustrating the rear sight assembly and the front sight assembly of the first embodiment of  FIGS. 1-12  mounted on a receiver supporting a machine gun; 
         FIG. 32  is a side elevation view of  FIG. 31 ; 
         FIG. 33  is a full front view of a front sight assembly and its connection to a one-piece base support cradle and trunnion adapter construction for attaching the front sight assembly to a receiver of a machine gun front view and containing an enlarged view A which is rotated 90 degrees; 
         FIG. 34  is a full right-side view of the front sight assembly of  FIG. 33 ; 
         FIG. 35  is an enlarged top view of a windage indicator and windage plate used in the front sight assembly of  FIGS. 33 and 34 ; 
         FIG. 36  is a top plan view illustrating the rear front assembly and the front sight assembly of the second embodiment of the present invention mounted on a receiver supporting a machine gun; and 
         FIG. 37  is a side elevation view of  FIG. 36 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A complete understanding of the present invention will be obtained from the following description taken in connection with the accompanying drawings, wherein like reference characters identify the parts throughout. For the purposes of the following description, the terms “above”, “below”, “top”, “bottom” “vertical”, “horizontal” and derivatives thereof refer to the invention as oriented in the drawings. 
     It is to be understood that the invention may assume alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings and described in the following specification are exemplary embodiments of the invention. Specific dimensions and other physical characteristics related to the embodiments disclosed herein are not considered to be limiting. 
       FIGS. 1-12  and  FIGS. 31-32  pertain to a first embodiment of a sighting system of the invention and may find particular application on an M2HB machine gun.  FIGS. 13   a - 30   b  pertain to a second embodiment of a sighting system of the invention and which find particular application on an M3 flexible machine gun.  FIGS. 33-35  illustrate an alternative construction for the second embodiment of  FIGS. 13   a - 30   b  wherein the base support cradle and the trunnion are a one-piece construction instead of a TIG welded construction. 
     Referring particularly to the first embodiment of the sighting system,  FIGS. 1-7   c  illustrate a rear sight assembly  10  and  FIGS. 9-12  illustrate a front sight assembly  12 .  FIGS. 31-32  illustrate the rear sight assembly  10  and the front sight assembly  12  being mounted on the top surface of a receiver R supporting a machine gun MG. 
     The rear sight assembly  10  of  FIGS. 1-6   a  include an aperture member  14 , a base support cradle  16  for supporting the aperture member  14  and a detent plunger spring assembly  18  connected to the aperture member  14  for registering and retaining the aperture member  14  in a fixed position within the base support cradle  16  and relative to a line-of-sight while a machine gun is being operated. As particularly shown in  FIGS. 3 ,  4  and  5 , the aperture member  14  is an L-shaped pivotal member having a first leg  20  and a second leg  24 , and as particularly shown in  FIGS. 1 and 2 , the first leg  20  has an aperture  22  and the second leg  24  has an aperture  26 . As can be appreciated from  FIGS. 1 and 2 , the aperture  22  is greater in diameter than that of aperture  26 , the latter of which is also referred to as a rear peep sight or a piggy back aperture. That is, the diameter of aperture  22  may be about ⅜ of an inch while that of aperture  26  may be about 3/16 of an inch. Aperture  22  may be used for 100 to 500 yard targets and the piggy back aperture  26  may be used for 500 to 1200 yard targets. 
     Still referring particularly to  FIGS. 1-6   a , aperture member  14  is pivotally mounted in base support cradle  16  via an elongated shaft  28  which extends through support members  30 ,  32  of the base support cradle  16 . As particularly shown in  FIGS. 1 ,  2 ,  3  and  6   a , the shaft  28  is a part of a windage screw assembly  34 . Elongated shaft  28  extends outwardly from the support member  32  as shown to the left of  FIGS. 1 and 2  and to the right of  FIG. 6   a  and extends through a knurled knob  36 , which, in turn, is secured to the end of elongated shaft  28  via a retaining roll pin  38 . Elongated shaft  28  has a threaded external portion indicated at  40  in  FIGS. 1 ,  2  and  6   a  which extends longitudinally through the aperture member  14  via a longitudinal opening  28   a  which is shown best in  FIGS. 7   a ,  7   b  and  7   c , and which engages with internal threads as shown at  42  in  FIGS. 1 ,  2 ,  6   a  and  FIG. 7   c . Knurled knob  36  is rotatable and is used to rotate the elongated shaft  28  and its threaded portion  40  for the lateral windage movement of the aperture member  14  along the threaded portion  40  of the elongated shaft  28  and within the base support cradle  16 . 
     Windage screw assembly  34  further includes a detent screw/plunger assembly  44  which is located in the knob  36  and a detent bushing member  46  shown best to the left of  FIGS. 1 and 2 , at the top of  FIG. 3  and to the right of  FIG. 6   a . Detent bushing member  46  has a plurality of recesses, one indicated at number  48  for receiving and retaining a plunger  50  of the detent screw/plunger assembly  44 , for preventing rotation of the knob  36  and the elongated shaft  28  of the windage screw assembly  34 . As shown in  FIGS. 1 ,  2 ,  3  and  6   a , the end of the elongated shaft  28  opposite knob  36  contains a screw head  52  which is part of the elongated shaft  28 . This screw head  52  holds and gives the windage screw assembly  34  a stronger lock up of the components of the windage screw assembly  34 . The lateral windage movement of the aperture member  14  along the threaded portion  40  of the elongated shaft  28  is about 3/16 inch to the right and about 3/16 inch to the left with reference to  FIGS. 1 ,  2  and  6   a  for a total movement of about ⅜ inch. The extent of the lateral windage movement of aperture member  14  is established via an indicator  53  ( FIG. 7   a ) on aperture member  14  and windage plate  55  ( FIG. 3 ) and alignment of indicator  53  with the lines numbered  5 - 0 - 5  on windage plate  55 , which is supported on base plate  70  ( FIG. 3 ). 
     As shown in  FIGS. 1 ,  7   a ,  7   b  and  7   c , aperture member  14  further includes a longitudinal opening  54 . As shown best in  FIG. 1 , longitudinal opening  54  receives the detent plunger spring assembly  18  which is operatively connected to the aperture member  14 . As best shown in  FIGS. 1 and 6   a , the detent plunger spring assembly  18  includes at least two opposed plunger members  56 ,  58  and a spring  60  which extends between and into the two plunger members  56 ,  58 . As best shown in  FIGS. 1 and 6   a  plunger member  56  is received in an opening  62  of support member  30  and plunger member  58  is received in an opening  64  of support member  32  of base support cradle  16 . Upon rotation of knob  36 , elongated shaft  28  is rotated to pivot the first leg  20  of the L-shaped member of aperture member  14  into the position of  FIG. 4  and then to pivot the second leg  24  of the L-shaped member of aperture member  14  into the position of  FIG. 5 , as indicated by arrows A 1  and A 2  in  FIG. 4 . During this pivotal movement of the L-shaped member of aperture member  14 , the plunger members  56 ,  58  are forced out of openings  62 ,  64  of support members  30 ,  32 , respectively, and inwardly via contact of the plunger members  56 ,  58  against the inner surfaces ( FIGS. 1 and 2 ) of the support members  30 ,  32  to compress spring  60 , and then plunger members  56 ,  58  are forced outwardly to engage similar openings  66 ,  68 , respectively, located in support members  30 ,  32  as indicated in  FIGS. 4 and 5 . The rounded end of detent plunger member  56 ,  58  has a radius of about 0.25 inch. 
     As best shown in  FIGS. 1-6   a , base support cradle  16  is mounted on the base plate  70  via a plurality of retaining screws  72 , four of which are shown in  FIG. 3 . 
       FIG. 6   b  shows that the detent plunger spring assembly  18  may also include a sleeve assembly  78  configured to receive the two opposed plunger members  56 ,  58  and the spring  60 . This arrangement may be used to repair or replace the detent plunger spring assembly  18 , illustrated in  FIG. 6   a , particularly if the inner surface of the longitudinal opening  54  of aperture member  14  ( FIGS. 7   a - 7   c ) wears after an extreme length of time and use. If a repair is needed, the sleeve assembly  78  is inserted around the plunger members  56 ,  58  and the spring  60 . The thickness of the sleeve assembly  78  will be such as to take up any clearance existing between the inner surface of the longitudinal opening  54  and the outer surfaces of plunger members  56 ,  58  due to prolonged periods of wear. 
       FIGS. 8   a  and  8   b  illustrate a further embodiment for a detent plunger spring assembly  80  which can be used as an alternative in the L-shaped member of aperture member  14  of  FIGS. 1-7   c . This detent plunger spring assembly  80  includes a guide rod  82 . Guide rod  82  has ends  84 ,  86  which have a reduced diameter compared to a middle portion  88  of guide rod  82 . As shown in  FIG. 8   a , each end  84 ,  86  has a spring  90 ,  92 , respectively, which fits into one of the two opposed plunger members  94 ,  96  respectively. Plunger members  94 ,  96  have a rounded end with a radius of about 0.25 inch. 
       FIGS. 9-12  illustrate the front sight assembly  12  for use with the rear sight assembly  10  of  FIGS. 1-7   c . Front sight assembly  12  includes an aperture member  98  having an upper arched portion  100  which defines a window  102  in which a sight post  104  extends upwardly. Front sight assembly  12  further includes a first base member  106  which is attached to the aperture member  98  as indicated by reference number  108  ( FIGS. 9-11 ) and which first base member  106  has a lower dovetail surface  110  as particularly shown in  FIGS. 11 and 12 . Front sight assembly  12  further includes a second base member  112  or a dovetail base fixedly secured with two ⅛ inch diameter retaining pins (not shown) through the front receiving trunnion on top of the machine gun MG as illustrated in  FIGS. 31 and 32 . Still referring to  FIGS. 11 and 12 , second base member  112  has a dovetail upper surface  114  that corresponds to that of and is slidable within the dovetail surface  110  of the first base member  106  for the lateral movement of the aperture member  98  and the first base member  106  relative to the longitudinal plane of the machine gun MG ( FIGS. 31 and 32 ). It is to be appreciated that the second base member  112  is fixedly secured via retaining pins to the front receiving trunnion located on top of the machine gun in a manner well known to those skilled in the art. 
     Still referring to  FIGS. 9 and 12 , the sight post  104  has an external threaded portion  116  which extends through aperture member  98 , through an elevation knob  118  and into first base member  106 . Elevation knob  118  has an internal threaded portion  120  ( FIG. 12 ) which engages the external threaded portion  116  of the sight post  104  for raising and lowering the sight post  104  within the window  102  of the aperture member  98  upon rotation of the elevation knob  118 . As sight post  104  travels in an upward direction and in a downward direction for its lowering within window  102 , it is guided by a pin retainer  122 , as shown best in  FIG. 12 . 
     Still referring to  FIGS. 9 and 12 , elevation knob  118  is located within an opening  124  in aperture member  98 . Elevation knob  118  has an upper shim-like surface  126  for abutting tightly against the upper surface of opening  124  and a lower surface having several notches or recesses, one shown at  128  in  FIGS. 9-12 . Located diametrically opposite the threaded portion  120  of sight post  104  are detent spring loaded plunger assemblies  130 ,  132  wherein plungers  134 ,  136  of each assembly  130 ,  132  engage in and are retained in one of the recesses  128  of elevation knob  118  upon rotation of the elevation knob  118  in the raising and lowering of sight post  104  in window  102 . This system, i.e., the two detent spring loaded plunger assemblies  130 ,  132  and the engagement of their plungers  134 ,  136  within the inverted V-shaped recesses  128  of the elevation knob  118  assures that sight post  104  remains in its desired position during operation of the machine gun MG. 
     As shown particularly in  FIG. 12 , the first base member  106  includes two sections  106   a ,  106   b  which are connected together via cap screws  138 ,  140 . Adjustment of the tensioning of the dovetail surface  110  of base member  106  against the dovetail surface  114  of the second base member  112  is achieved through tightening or loosening of the two cap screws  138 ,  140  to either separate or bring the two sections  106   a ,  106   b  of first base member  106  together. 
       FIGS. 13   a - 30   b  pertains to a second embodiment of a sighting system of the invention, which generally may be found on an M3 flexible  50  caliber machine gun.  FIGS. 13   a - 18   b  pertains to a fixed rear sight assembly  142  which generally does not provide for windage lateral movement or adjustment of an aperture.  FIGS. 19-24   b  pertain to a rear sight assembly  144  which does include an adjustment mechanism for windage lateral movement of an aperture.  FIGS. 25-30   a  pertain to a front sight assembly  146  which may be used either with the fixed rear sight assembly  142  of  FIGS. 13-17  or with the rear sight assembly  144  of FIGS.  19 - 23  and can be mounted on the top rear section of the receiver R supporting the machine gun MG as illustrated with respect to the first embodiment shown in  FIGS. 36 and 37 . 
     The fixed rear sight assembly  142  of  FIGS. 13   a - 17  include an aperture member  148 , a base support cradle  150  for supporting the aperture member  148 , and a detent plunger spring arrangement  152  ( FIG. 13   a ) which is operatively connected to the aperture member  148  for registering and retaining aperture member  148  in a fixed position within the base support cradle  150  and relative to a line-of-sight while the machine gun is being operated. As particularly shown in  FIGS. 15 and 16 , the aperture member  148  is an L-shaped pivotal member having a first leg  151  and a second leg  154 , and as particularly shown in  FIGS. 13   a ,  15  and  17 , the first leg  151  has a fixed center aperture  156 . As shown in  FIGS. 15 and 16 , the second leg  154  is relatively shorter than that of first leg  151  and does not have an aperture. Still referring to  FIGS. 15 and 17 , aperture member  148  also includes radial elements  158 ,  160  and  162 , each having a pointed end  158   a ,  160   a , and  162   a , respectively, pointing toward aperture  156 . 
     Referring to  FIGS. 13   a  and  14 - 17 , aperture member  148  is pivotally mounted in base support cradle  150  via an elongated shaft  164  which extends through support members  166 ,  168  ( FIGS. 13   a  and  17 ) of the base support cradle  150 . As particularly shown in  FIGS. 13   a  and  17 , the elongated shaft  164  is a part of a retaining screw assembly  170  which retains and prevents movement of aperture member  148  within base support cradle  150 . As shown best in  FIG. 17 , elongated shaft  164  has a threaded end portion  164   a  which extends into and engages with a threaded portion  168   a  of support member  168  of the base support cradle  150 , and bushing spacers  172 ,  174  are mounted onto elongated shaft  164  to space and then retain aperture member  148  in position relative to the inner surfaces of support members  166 ,  168  of base support cradle  150 . 
     As best shown in  FIG. 13   a , the detent plunger spring assembly  152  includes a first detent plunger spring assembly  152   a  and a second detent plunger spring assembly  152   b . Aperture member  148  further includes a longitudinal opening  176  in first leg member  151  and a longitudinal opening  178  in second leg member  154  for receiving the detent plunger spring assemblies  152   a ,  152   b , respectively, of the fixed rear sighting assembly  142 . Detent plunger spring assemblies  152   a ,  152   b  are operatively connected to aperture member  148 . 
     As best shown in  FIG. 13   a , the detent plunger spring assemblies  152   a ,  152   b  have two opposed plunger members  180 ,  182  and a spring  184  which extends between and into the two plunger members  180 ,  182 . As best shown in  FIGS. 13   a  and  17  plunger member  180  of detent plunger spring assembly  152   a  is received in an opening  186  and plunger member  180  of detent plunger spring assembly  152   b  is received in an opening  188  of support member  166 . Plunger member  182  of detent plunger spring assembly  152   a  is received in an opening  190  and plunger member  182  of detent plunger spring assembly  152   b  is received in an opening  192  of support member  168  of the base support cradle  150  when aperture member  148  is in its position illustrated in  FIGS. 13   a ,  14 ,  15  and  17 . However, when aperture member  148  is pivoted 90 degrees as illustrated in  FIG. 16 , plunger member  180  of detent plunger spring assembly  152   a  is received in an opening  194  (shown best in  FIGS. 14 and 15 ) of support member  166 , and the plunger member  182  of detent plunger spring assembly  152   a  is received in an opening  196  (shown best in  FIGS. 14 and 15 ) of support member  168 ; whereas plunger members  180 ,  182  of detent plunger spring assembly  152   b  are received in openings  186 ,  190  of support members  166  and  168 , respectively. 
     As will be appreciated, detent plunger spring assemblies  152   a ,  152   b  are constructed and operate similarly to the detent plunger spring assembly  18  of  FIGS. 1-6   a . Pivotal movement of aperture member  148  is generally done manually and is generally pivoted in the position of  FIG. 16  for stowing purposes. The two detent plunger spring assemblies  152   a ,  152   b  securely retain and maintain aperture member  148  in an operative position illustrated in  FIGS. 13   a ,  14 ,  15  and  17  and in an inoperative, stowing position illustrated in  FIG. 16 . 
     As best shown in  FIG. 14 , base support cradle  150  is mounted to a base plate  198  via a plurality of retaining screws  200 , four of which are shown in  FIG. 14 ; 
       FIG. 13   b  shows that the detent plunger spring assemblies  152   a ,  152   b  may also include a sleeve assembly  202  configured to receive the two opposed plunger members  180 ,  182  and spring  184 . This arrangement may be used to repair or replace the detent plunger spring assemblies  152   a ,  152   b  of the embodiment of  FIGS. 13   a  and  14 - 17 , particularly if the inner surface of the longitudinal openings  176 ,  178  of aperture member  148  ( FIGS. 15 and 16 ) wears after excessive use. If a repair is needed, sleeve assembly  202  may be inserted around plunger members  180 ,  182  and spring  184  of each detent plunger spring assembly  152   a ,  152   b . The thickness of sleeve assembly  202  will be such as to take up any clearance existing between the inner surface of the longitudinal openings  176 ,  178  and the outer surfaces of plunger members  180 ,  182  due to excessive use. 
       FIGS. 18   a  and  18   b  illustrate a further embodiment of a detent plunger spring assembly  204  which can also be used in aperture member  148  as an alternative for the detent plunger spring assemblies  152   a ,  152   b  of  FIG. 13   b . Assembly  204  includes a guide rod  206  having ends  208 ,  210  with a reduced diameter compared to a middle portion  212 . As shown in  FIG. 18   a , each end  208 ,  210  has a spring  214 ,  216 , respectively, which fits into one of the two opposed plunger members  218 ,  220 , respectively. Plunger members  218 ,  220  have a rounded end with a radius of about 0.25 inch. 
     As stated hereinabove,  FIGS. 19   a - 24   b  pertain to a movable rear sight assembly  144  ( FIGS. 19   a  and  20 - 23 ) which can be used as an alternative to the fixed rear sight assembly of  FIGS. 13   a - 18   b  in the second embodiment of the sighting system of the invention. 
     As shown in  FIGS. 19   a  and  23 , the movable rear sight assembly  144  includes an aperture member  222 , a base support cradle  224  for supporting the aperture member  222  and a detent plunger spring arrangement  226  which is operatively connected to the aperture member  222  for registering and retaining aperture member  222  in a fixed position within base support cradle  224  and relative to a line-of-sight when machine gun MG is operated. As particularly shown in  FIGS. 21 and 22 , aperture member  222  is an L-shaped pivotal member having a first leg  228  and a second leg  230 , and as particularly shown in  FIGS. 19   a  and  23 , first leg  228  has a fixed center aperture  232 . As shown in  FIGS. 21 and 22 , the second leg  230  is relatively shorter than that of first leg  228  and does not have an aperture. Still referring to  FIGS. 19   a  and  23 , aperture member  222  also includes radial elements  234 , each having a pointed end  234   a  that points inwardly toward the fixed center aperture  232 . 
     Referring particularly to  FIGS. 19   a ,  21 ,  22  and  23 , aperture member  222  is pivotally mounted in base support cradle  224  via an elongated shaft  236  which extends through support members  238 ,  240  ( FIGS. 19   a  and  23 ) of base support cradle  224 . As shown in  FIGS. 19   a  and  23 , the elongated shaft  236  is a part of a retaining screw assembly. 
     Elongated shaft  236  extends outwardly from the support member  238  as shown to the right of  FIG. 19   a  and to the left of  FIG. 23  and extends through a knurled knob  243 , which, in turn, is secured to the end of shaft  236  via a retaining roll pin  244 . Shaft  236  has a threaded external portion indicated at  246  in  FIG. 23  which extends longitudinally through the aperture member  222  via a longitudinal opening  248 , which is shown best in  FIG. 23 , and which engages with internal threads therein as shown at  250  in  FIG. 23 . Knob  243  is rotatable and is used to rotate the elongated shaft  236  and its threaded portion  246  for the lateral windage movement of aperture member  222  along the threaded portion  246  of elongated shaft  236  and within base support cradle  224 . 
     Referring particularly to  FIGS. 19   a ,  20  and  23 , windage screw assembly  237  further includes a detent screw/plunger assembly  252 , which is located in knob  243 , and a bushing member  254 . Bushing member  254  has a plurality of recesses, one indicated at number  254   a  for receiving and retaining a plunger  252   a  of the detent screw/plunger assembly  252  for preventing rotation of the knob  243  and the elongated shaft  236  of windage screw assembly  237 . As shown particularly to the left of  FIG. 19   a  and to the right of  FIG. 23 , the retaining screw assembly includes a screw head  241  located on the elongated shaft  236  opposite knob  243 . The retaining screw head  241  holds the windage screw assembly  237  and gives a stronger lock-up of the components of the windage screw assembly  237 . The total lateral windage movement of the aperture member  222  along the threaded portion  246  of the elongated shaft  236  is about ⅜ of an inch in both directions, i.e., about 3/16 inch to the right and about 3/16 inch to the left of  FIGS. 19   a  and  23 . 
     As best shown in  FIG. 19   a , the detent plunger spring arrangement  226  includes a first detent plunger spring assembly  226   a  and a second detent plunger spring assembly  226   b . As shown in  FIGS. 21 and 22 , aperture member  222  further includes a longitudinal opening  228   a  in first leg member  228  and a longitudinal opening  230   a  in second leg member  230  for receiving the detent plunger spring assemblies  226   a ,  226   b , respectively, of the rear sight assembly  144 . Detent plunger spring assemblies  226   a ,  226   b  are operatively connected to the aperture member  222 . 
     As best shown in  FIGS. 19   a  and  23 , detent plunger spring assemblies  226   a ,  226   b  have two opposed plunger members  258 ,  260  and a spring  262  which extends between and into the plunger members  258  and  260 . As best shown in  FIGS. 19   a  and  23 , plunger member  258  of detent plunger spring assembly  226   a  is received in an opening  264  and plunger member  258  of detent plunger spring assembly  226   b  is received in an opening  266  of support member  238 . Plunger member  260  of detent plunger spring assembly  226   a  is received in an opening  268  and plunger member  260  of detent plunger spring assembly  226   b  is received in an opening  270  of support member  240  of the base support cradle  224  when aperture member  222  is in its position illustrated in  FIGS. 19   a ,  20 ,  21  and  23 . However, when aperture member  222  is pivoted 90 degrees, as illustrated in  FIG. 22 , the plunger members  258 ,  260  of detent plunger spring assembly  226   a  are received in openings  272 ,  274  of support members  238 ,  240 , respectively ( FIGS. 21 and 22 ), and the plunger members  258 ,  260  of detent plunger spring assembly  226   b  are received in openings  264 ,  268  of support members  238 ,  240 , respectively. 
     As will be appreciated, detent plunger spring assemblies  226   a ,  226   b  are constructed and operate similarly to detent plunger spring assembly  18  of the embodiment of  FIGS. 1-6   a . Pivotal movement of aperture member  222  is generally done manually and is generally pivoted in the position of  FIG. 22  for stowing purposes. The two detent plunger spring assemblies  226   a ,  226   b  securely retain and maintain aperture member  222  in an operative position illustrated in  FIGS. 19   a ,  20  and  21  and in an inoperative, stowing position illustrated in  FIG. 22 . 
     As best shown in  FIGS. 19   a  and  20 - 23 , base support cradle  224  is mounted to a base plate  280  via a plurality of retaining screws  282 , four of which are shown in  FIG. 20 . 
       FIG. 19   b  shows that detent plunger spring assemblies  226   a  and  226   b  may also include a sleeve assembly  284  configured to receive the two opposed plunger members  258 ,  260  and spring  262 . This arrangement may be used to repair or replace the detent plunger spring assemblies  226   a ,  226   b  illustrated in the embodiment of  FIGS. 19   a  and  20 - 23 , particularly if the inner surface of the longitudinal openings  228   a ,  230   a  of aperture member  222  ( FIGS. 21 and 22 ) wears after excessive use. If a repair is needed, sleeve assembly  284  is inserted around plunger members  258 ,  260  and spring  262 . The thickness of sleeve assembly  284  will be such as to take up any clearance existing between the inner surface of longitudinal openings  228   a ,  230   a  and the outer surfaces of plunger members  258 ,  260  due to excessive wear. 
       FIGS. 24   a  and  24   b  illustrate a further embodiment for a detent plunger spring assembly  286  which can also be used in aperture member  222  as a replacement for detent plunger spring assemblies  226   a ,  226   b  of  FIGS. 19   a - 23 . Detent plunger spring assembly  286  includes a guide rod  288  having ends  290 ,  292  with a reduced diameter compared to a middle portion  294 . As shown in  FIG. 24   a , each end  290 ,  292  has a spring  296 ,  298 , respectively, which fits into one of the two opposed plunger members  300 ,  302 , respectively. Plunger member  300 ,  302  have a rounded end with a radius of about 0.25 inch. 
     As stated hereinabove,  FIGS. 25-30   a  illustrate the front sight assembly  146  for use with the fixed rear sight assembly  142  of  FIGS. 13   a - 18   b  or the movable rear sight assembly  144  of  FIGS. 19   a - 24   b . It is to be appreciated that some of the reference numbers may not be included in  FIGS. 25-30   a  for clarity purposes. As particularly shown in  FIG. 25 , front sight assembly  146  includes an aperture member  306  having a circular aperture  308  which has a window  310  and a center post  312  which extends upwardly in window  310 . Front sight assembly  146  further includes an elongated threaded shaft post  314  mounted in and connected to aperture member  306  at its one end and extending into a stop plate  316  and secured in stop plate  316  via a retaining screw  318 . Mounted around elongated threaded shaft post  314  are a locking nut knob  320  and a washer  322  which may be bronze or brass. Locking nut knob  320  has an inner surface with threads that correspond to and engage the threads on elongated threaded shaft post  314 . Locking nut knob  320  locks aperture member  306  in a fixed position. Aperture member  306  is rotated manually in a clockwise or a counter-clockwise direction about the longitudinal axis B in  FIG. 25  to raise or lower the aperture member  306  and the locking nut knob  320  is then rotated to lock or fix aperture member  306  in position. A housing retainer  324  contains two V-shaped head plunger assemblies  326 , one located diametrically opposite to the other relative to elongated threaded shaft  314  and each having a V-shaped head  328 . The elongated threaded shaft post  314  has at least two elongated V-troughs  330 , each located on diametrically opposite sides along the elongated threaded shaft post  314 . Each V-shaped head  328  of V-shaped head plunger assembly  326  moves into and engages an elongated V-trough  330  of elongated threaded shaft post  314 , as shown best in  FIGS. 27-29 . Each plunger assembly  326  has a spring  326   a  which forces the V-shaped head  328  into the elongated V-trough  330  to lock the elongated threaded shaft  314  in a desired elevational position. Plunger assemblies  326  operate upon each 180 degree turn of the aperture member  306 . That is, plunger assemblies  326  are forced outwardly when they are rotated and then snap inwardly into the V-trough  330  to lock elongated shaft  314  at the end of each 180 degree turn. When aperture member  306  is in its desired position, locking nut knob  320  is turned until it abuts against washer  322 . The enlarged “A” view of  FIG. 25  is a top view and illustrates the engagement of the V-shaped head  328  of one plunger assembly  326  in an elongated V-shaped trough  330  located on opposite sides of elongated threaded shaft  314  ( FIGS. 27-29 ). 
     As best shown in  FIGS. 27 ,  28  and  29 , elongated threaded shaft post  314  is mounted through a C-shaped member  336 . Aperture member  306  of front sight assembly  146  is connected via the pivotal C-shaped member  336 , which in turn is connected to a base support cradle  334 . Base support cradle  334  preferably is TIG welded to a trunnion adapter  338  shown in  FIGS. 26 ,  28  and  29  for attachment of the front sight assembly  146  onto a trunnion of the receiver R of the machine gun MG through means well known to those skilled in the art and as shown in  FIGS. 36 and 37 . The C-shaped member  336  contains the housing retainer  324  for housing the V-shaped head plunger assemblies  326 . 
     Similar to the rear sight assemblies  142 ,  144  of this second embodiment, the front sight assembly  146  also has a detent plunger spring arrangement including a first detent plunger spring assembly  340   a  and a second detent plunger spring assembly  340   b  located at about a 90 degree angle relative to each other in pivotal C-shaped member  336  and a windage screw knob assembly  342 , which operate similar to those described hereinabove. Detent plunger spring assemblies  340   a ,  340   b  include two opposed plunger members  344 ,  346  and a spring  348 . Each plunger assembly  340   a ,  340   b  extends in a longitudinal opening  350 ,  352 , respectively, located in the pivotal C-shaped member  336 , which is attached to aperture member  306  with the two plunger members  344 ,  346  of each plunger assembly  340   a ,  340   b  (numbered in  FIG. 30   a ) engage openings  354 ,  356  located in the two support members  334   a ,  334   b  of base support cradle  334 . However, when aperture member  306  is pivoted 90 degrees as illustrated in  FIG. 29 , plunger members  344 ,  346  of detent plunger spring assembly  340   a  are received in openings  358  ( FIG. 28 ) of support members  334   a ,  334   b , respectively ( FIG. 29 ), and plunger members  344 ,  346  of detent plunger spring assembly  340   b  are received in openings  354  of support members  334   a ,  334   b , respectively. Referring particularly to  FIGS. 27-29 , as can be appreciated, openings  354 ,  356  in support members  334   a ,  334   b  are located at a 90 degree angle relative to each other and openings  356  and  358  are located at a 90 degree angle relative to each other as best shown in  FIG. 27  so that plunger members  344 ,  346  of the detent plunger spring assemblies  340   a ,  340   b  may engage the appropriate openings  354 ,  356  and  358  when aperture member  306  of the front sight assembly  146  is pivoted to the right as indicated by the arrow in  FIG. 28  for its stowing position and for an inoperative position of the machine gun MG or when aperture member  306  of front sight assembly is pivoted upright in its operative position and for an operative position of the machine gun MG, as illustrated in  FIGS. 25 ,  26 ,  27 ,  28  and  30   a.    
     As shown best in  FIG. 25 , the windage screw knob assembly  342  of front sight assembly  146  includes a knurled knob  362  mounted via a retaining roll pin  364  onto an elongated shaft  366  extending through the two support members  334   a ,  334   b  of base support cradle  334  and the lower pivotal C-shaped member  336  of aperture member  306 . Elongated shaft  366  has a threaded portion which engages with the inner threads of a further longitudinal opening  370  of lower pivotal C-shaped member  336  of aperture member  306 . Rotation of knob  362  rotates the elongated shaft  366  which causes the pivotal C-shaped member  336  and aperture member  306  to move transversely or laterally for windage purposes. As stated hereinabove, this windage screw knob assembly  342  is similar to those described hereinabove for the rear sight assembly  144  of  FIGS. 19   a - 23 . Windage screw knob assembly  342  is held in a fixed position via a spring assembly  343  which engages one of the V-shaped recesses in V-notched plate  345 , as indicated particularly in  FIG. 26 . 
       FIG. 30   b  shows that the detent plunger spring assemblies  340   a ,  340   b  of the front sight assembly  146  may also include a sleeve assembly  372  configured to receive the two plunger members  344 ,  346  and the spring  348 . This arrangement may be used to repair or replace the detent plunger spring assemblies  340 ,  340   b  illustrated in  FIGS. 25-30   a  particularly if the inner surface of the longitudinal openings  350 ,  352  of pivotal C-shaped member  336  ( FIG. 27 ) wears after excessive use. If a repair is needed, sleeve assembly  372  may be inserted around plunger members  344 ,  346  and spring  348 . The thickness of sleeve assembly  372  will be such as to take up any clearance existing between the inner surface of the longitudinal openings  350 ,  352  and the outer surfaces of plunger members  344 ,  346  of detent plunger spring assemblies  340   a ,  340   b  due to wear. It is also to be appreciated that the detent plunger spring assembly  204  of  FIGS. 18   a  and  18   b  may also be used in the rear sight assembly  144  instead of the detent plunger spring assemblies  340   a ,  340   b  described herein. 
     Further, it is to be appreciated that the radial elements  158 ,  160  and  162  of rear sight assembly  142  and radial elements  234  of movable rear sight assembly  144 , depending on which rear sight assembly is being used in conjunction with the front sight assembly  146 , may be aligned with the circular aperture  308  of front sight assembly  146  so that aperture  156  of rear sight assembly  142  or aperture  232  of movable rear sight assembly  144  can be aligned with the center post  312  of the front sight assembly  146 . Such alignment is achieved through rotation of the windage screw knob assembly of the movable rear sight assembly  144  and the front sight assembly  146  and through the rotation of locking nut knob  320  of the front sight assembly  146  by turning locking nut knob  320  counter-clockwise then grasping the top section of the aperture member  306  and turning it clockwise or counter-clockwise. On each half rotation, the V-shaped head plunger assemblies  326  lock the aperture member  306  in place thereby positioning the aperture member  306  perpendicularly to the receiver R of the machine gun MG ( FIGS. 36 and 37 ) on each turn until a correct or desired elevation of aperture member  306  is achieved. Stop plate  316  at the bottom of aperture member  306  keeps aperture member  306  from exiting the pivotal C-shaped member  336  in an upward direction. The total adjustable travel for aperture member  306  is about 0.25 inch. When aperture member  306  is in its desired elevational positioning, then locking nut knob  320  is rotated until it engages washer  322 . Retaining of the rear sight assemblies  142  and  144  and the front sight assembly  146  is achieved through their respective detent spring plunger assemblies as disclosed hereinabove. 
       FIGS. 33 and 34  illustrate a front sight assembly  400  which is similar to the front sight assembly  146  of  FIGS. 25-30   a  except that an aperture member  402  is connected via a pivotal C-shaped member  404  to a one-piece base support cradle-trunnion adapter arrangement  406 , which is used for the attachment of the front sight assembly  400  onto a trunnion of a receiver R of the machine gun MG of  FIGS. 36 and 37  through means well known to those skilled in the art. It is to be appreciated that in the embodiment of  FIGS. 25-30   a , the base support cradle  334  and the trunnion adapter  338  are two separate pieces where the base support cradle  334  is TIG welded to the trunnion adapter  338 ; whereas in the embodiment of  FIGS. 33 and 34 , the base support cradle  408  and the trunnion adapter  410  constitute a one-piece construction indicated generally at reference number  406 . This one-piece construction arrangement  406  may be machined out of 4140 chrome steel molly (mil. Spec). The other components of the front sight assembly  400  of  FIGS. 33 and 34  are similar to the embodiment of  FIGS. 25-30   a.    
     More particularly, and with reference to  FIG. 33 , front sight assembly  400  includes aperture member  402  having an upper circular aperture  412  having a window  414  and a center post  416  extending upwardly in window  414 . Front sight assembly  400  further includes an elongated threaded shaft post  418  mounted in and connected to aperture member  402  at its one end and extending into a stop plate  420  and secured in stop plate  420  via a retaining screw  422 . Mounted around elongated threaded shaft post  418  are a locking nut knob  424  and a washer  426  which may be bronze or brass. Locking nut knob  424  has an inner surface with threads that correspond to and engage the threads on shaft  418 . Locking nut knob  424  locks aperture member  402  in a fixed elevational position. Aperture member  402  is rotated manually in a clockwise or a counter-clockwise direction to raise and lower aperture member  402  and locking nut knob  424  is then rotated to lock or fix aperture member  402  in position. 
     A housing retainer  428  contains two V-shaped head plunger assemblies  430  (best shown in the enlarged view A of  FIG. 33 ), one located diametrically opposite to the other relative to shaft  418  and each having a V-shaped head  432 . The elongated threaded shaft post  418  has at least two elongated V-troughs  434 , each located on diametrically opposite sides of elongated threaded shaft post  418 . The V-shaped head  432  of the V-shaped head plunger assemblies  430  moves into and engages an elongated V-trough  434  of elongated threaded shaft  418 , as shown best in  FIG. 34 . Each plunger assembly  430  has a spring  437  which forces the V-shaped head  432  into the elongated V-trough  434  to lock the elongated threaded shaft  418  in a desired elevational position. Plunger assemblies  430  operate upon each 180 degree turn of the aperture member  402 . That is, plunger assemblies  430  are forced outwardly when aperture member  402  along with shaft  418  is rotated, preferably manually, and then plunger assemblies  430  snap inwardly into the V-troughs  434  to lock the elongated shaft  418  at the end of each 180 degree turn in either a clockwise or counter-clockwise rotation of aperture member  402 . When aperture member  402  is rotated into its desired position, locking nut knob  424  is turned, preferably manually, until locking nut knob  424  abuts against washer  426 . The enlarged A view of  FIG. 33  is a top view and illustrates the engagement of the V-shaped head  432  of one plunger assembly  430  in an elongated V-shaped trough  434  located on opposite sides of elongated threaded shaft  418  ( FIG. 34 ). 
     As shown in  FIGS. 33 and 34 , and as discussed hereinabove, elongated threaded shaft post  418  is mounted through a C-shaped member  404 . Aperture member  402  of front sight assembly  400  is connected via the elongated threaded shaft post  418  to the pivotal C-shaped member  404 , which, in turn, is connected to the one-piece base support cradle-trunnion adapter arrangement  406 . The cradle-trunnion adapter arrangement  406  is used for the attachment of the front sight assembly  400  onto a trunnion (not shown) of a receiver R of the machine gun MG ( FIGS. 36 and 37 ) through means well known to those skilled in the art. The C-shaped member  404  contains the housing retainer  428  for the V-shaped head plunger assemblies  430 . 
     As best shown in  FIG. 33 , front sight assembly  400  also has a detent plunger spring arrangement including a first detent plunger spring assembly  436  and a second detent plunger spring assembly  438  located at about a 90 degree angle relative to each other in pivotal C-shaped member  404  and a windage screw knob assembly  440 , which operates similar to those described hereinabove. Detent plunger spring assemblies  436 ,  438  include two opposed plunger members  442 ,  444  and a spring  446 . Each plunger assembly  436 ,  438  extends in a longitudinal opening  448 ,  450 , respectively (best shown in  FIG. 34 ), located in the pivotal C-shaped member  404 , which is attached to aperture member  402  with the two plunger members  442 ,  444  of each plunger assembly  436 ,  438  (numbered in  FIG. 33 ) engaging openings  448 ,  450  located in base support cradle  408 . However, when aperture member  402  is pivoted 90 degrees as indicated by the arrow in  FIG. 34 , plunger members  442 ,  444  of detent plunger spring assembly  436  are received in openings  452  of base support cradle  408 , and plunger members  442 ,  444  of detent plunger spring assembly  438  are received in openings  448  of base support cradle  408 , respectively. Openings  448  and  452  in base support cradle  408  are located at a 90 degree angle relative to each other and openings  448  and  450  are located at a 90 degree angle relative to each other as best shown in  FIG. 34  so that plunger members  442 ,  444  of the detent plunger spring assemblies  436 ,  438  may engage the appropriate openings  448 ,  450  and  452  when aperture member  402  is pivoted to the right as indicated by the arrow in  FIG. 34  for its stowing position or when aperture member  402  is pivoted upright in its operative position illustrated in  FIGS. 33 and 34 . 
     As shown best in  FIG. 33 , the windage screw knob assembly  440  includes a knurled knob  454  mounted via a retaining roll pin  456  onto an elongated shaft  458  extending through base support cradle  408  and C-shaped member  404  supporting aperture member  402 . Elongated shaft  458  has a threaded portion which engages with the inner threads of a further longitudinal opening  460  ( FIG. 34 ) of C-shaped member  404 . Rotation of knob  454  rotates the elongated shaft  458  which causes the C-shaped member  404  and aperture member  402  to move transversely or laterally for windage purposes. As stated hereinabove, this windage screw knob assembly  440  is similar to those described hereinabove for the rear sight assembly  144  of  FIGS. 19   a - 23 . Windage screw knob assembly  440  is held in a fixed position via a spring assembly  462  which engages one of the V-shaped recesses in V-notched plate  464  as indicated particularly in  FIG. 33 . 
     Referring particularly to  FIGS. 33 ,  34  and  35 , a windage indicator  466  with a pointer  468  ( FIG. 35 ) is attached to the C-shaped member  404  via screws  470  as best shown in  FIG. 35 . A trunnion cover  471  with a windage scale plate  472  is associated with the windage indicator  466  as best shown in  FIG. 35 . As indicated in  FIG. 34 , trunnion cover  471  and windage scale plate  472  are attached to the top of trunnion of the receiver R of the machine gun MG ( FIGS. 36 and 37 ). As is apparent, the extent of the transverse or lateral movement of C-shaped member  404  and aperture member  402  to the right or left of the longitudinal plane of receiver R and therefore machine gun MG will be indicated via pointer  468  of windage indicator  466  and its alignment with the lines numbered  5 - 0 - 5  on windage scale plate  472 . It is to be appreciated that this windage indicator  466  and windage scale plate  472  may be used with the other embodiments of the invention. 
     It is to be further appreciated that optionally the front and rear sight assemblies disclosed herein may be used with existing front and rear sight assemblies. That is, optionally front sight assembly  12  of  FIGS. 31 and 32  may in some instances be used in conjunction with additional designs for a rear sight assembly other than the rear sight assembly  10  disclosed herein, and optionally, rear sight assembly  10  of  FIGS. 31 and 32  may in some instances be used in conjunction with additional designs for a front sight assembly other than the front sight assembly  12  disclosed herein. Additionally, optionally, front sight assemblies  146  and  400  of  FIGS. 36 and 37  may in some instances be used in conjunction with additional designs for a rear sight assembly other than the rear sight assemblies  142  and  144  disclosed herein, and optionally, rear sight assemblies  142  and  144  of  FIGS. 36 and 37  may in some instances be used in conjunction with additional designs for a front sight assembly other than the front sight assemblies  146  and  400  disclosed herein. 
     Although the present invention has been described with reference to particular embodiments of a sighting system for particular use on a machine gun, those skilled in the art may make modifications and alterations to the present invention without departing from the spirit and scope of the invention. Accordingly, the foregoing detailed description is intended to be illustrative rather than restrictive. The invention is defined by the appended claims, and all changes to the invention that fall within the meaning and range of equivalency of the claims are embraced within their scope.