Abstract:
The rifling in a barrel of a firearm is modified in proximity to the muzzle to reduce the radial difference between the land diameter and the groove diameter to urge a bullet to expand fully into the grooves and ensure rotation of the bullet about its longitudinal axis and without any lateral velocity component. Preferably, the land diameter is increased to a diameter somewhat less than the groove diameter but in the alternative the groove diameter may be reduced to a diameter somewhat more than the land diameter.

Description:
The present invention claims priority to the subject matter disclosed in a provisional application entitled “GUN BARREL RIFLING” filed May 21, 1999 and assigned Ser. No. 60/135,294 directed to an invention made by the present inventor. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to gun barrel rifling and, more particularly, to rifling having a varying radial spacing between the surface of the lands and the bottom surfaces of the interleaved grooves as a function of distance along the barrel. 
     2. Description of Related Art 
     A conventional gun barrel of a firearm includes a bore having rifling formed therein. That is, the rifling usually includes a plurality of helical, sometimes referred to as spiral, radially inwardly facing lands with interleaved grooves. The surfaces of these lands are usually arcuate corresponding in curvature with the respective radius from the axis of the bore. Other configurations of the surfaces of the lands are known. Each of the grooves usually includes a bottom surface also arcuate as a function of the radius from the axis of the bore. Other configurations for the bottom surfaces of the grooves are known. Generally, the sides interconnecting the lands with the bottom surfaces of the grooves are essentially radially aligned. However, other surface configurations interconnecting the lands with the bottom surfaces of the grooves are known. 
     Typically, six lands and corresponding six grooves are equiangularly displaced about the bore of the barrel. For purposes of terminology, the term land diameter means the distance from one land to a radially opposing land. The groove diameter is the distance from the bottom surface of one groove to the bottom surface of a radially opposing groove. The diameter (caliber) of the projectile or bullet fired through a barrel usually corresponds with the groove diameter. To define the distance between the surface of a land and the longitudinal axis of the bore and the distance between the bottom surface of a groove and the longitudinal axis of the bore, the terms land radius and groove radius, respectively, may be used. 
     As a firearm is discharged, the gases generated within the casing propel or fire the bullet through the barrel. Because the land diameter is less than the diameter of the usually cylindrical part of the bullet, the lands will engrave corresponding channels in the bullet. As the bullet travels toward the muzzle, the channels interacting with the respective lands, will cause the bullet to rotate about its longitudinal axis at a rate commensurate with the helix (or spiral) of the lands and the velocity/acceleration of the bullet traveling through the bore of the barrel. 
     Upon close inspection of bullets fired through a barrel, it has been learned that the grooves formed in the bullet by the respective lands are often not of the same uniform depth. Such lack of uniformity of depth suggests that the longitudinal axis of the bullet is not coincident with the longitudinal axis of the bore of the barrel. Without such coincidence, the bullet will be laterally displaced as it is longitudinally displaced during its travel through the bore. Assuming that the center of gravity of the bullet is located on its longitudinal axis, the asymmetric depth of the grooves in the bullet will cause the center of gravity of the bullet to move in a helix (spiral) as the bullet translates through the bore. Upon discharge of the bullet from the muzzle, the spinning motion of the bullet will have a lateral velocity component of some degree. Such lateral velocity component will result in inaccuracy of travel as a function of the degree of lateral velocity component present. As it is unlikely that any two consecutively fired bullets will have exactly the same degree of lateral velocity component, different paths or trajectories will be followed by each bullet. Such different trajectories will result in non-correspondence of the bullets striking the same point on a target, assuming all other variables of windage, etc. being constant. 
     SUMMARY OF THE INVENTION 
     As a projectile or bullet begins to travel down the bore of a gun barrel upon discharge of a firearm, a plurality of the lands tapering radially inwardly engrave or form channels in the bullet to guide the bullet and cause it to spin as a function of the helix (or spiral) of the rifling. The pressure of the gases behind the bullet in combination with the resistance to forward travel of the bullet induced by the lands of the rifling, as well as the inertia of the bullet, causes the material of the bullet to expand radially to a greater or lesser degree into the grooves interleaving the lands. Toward the muzzle of the barrel, the ratio of the land diameter to the groove diameter is increased to induce the material of the bullet to come into uniform contact with the bottom surface of each of the grooves to centralize the bullet within the bore and to ensure that the spin of the bullet is essentially about its longitudinal axis and hence about the center of gravity of the bullet to minimize or eliminate any lateral velocity component of the bullet. To increase the ratio of the land diameter to the groove diameter, the lands may taper radially outwardly toward the bore surface, and hence toward the bottom surface of the grooves, or the bore may be reduced to bring the groove diameter toward the land diameter. 
     It is therefore a primary object of the present invention to reduce any lateral velocity component of a projectile as it translates through the rifling in a barrel of a firearm. 
     Another object of the present invention is to induce the material of a projectile traveling through a rifled bore to become supported by the bottom surfaces of the grooves interleaving the lands in the bore. 
     Still another object of the present invention is to increase the ratio of the land diameter to the groove diameter toward the muzzle of a rifled barrel of a firearm. 
     A further object of the present invention is to provide varying depth grooves in a rifled barrel of a firearm to urge a projectile discharged from the barrel to spin about its longitudinal axis and without any lateral velocity component. 
     A still further object of the present invention is to provide a rifled barrel of a firearm having an increased land diameter in proximity to the muzzle. 
     A yet further object of the present invention is to provide a rifled barrel of a firearm having a reduced groove diameter in proximity to the muzzle. 
     A yet further object of the present invention is to provide a method for reducing the lateral velocity component of a projectile traveling through the barrel rifling of a firearm. 
     These and other objects of the present invention will become apparent to the those skilled in the art as a description thereof proceeds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described with greater specificity and clarity with reference to the following drawings, in which: 
     FIG. 1 illustrates a cross-section of a rifled bore of a firearm and wherein the lands and grooves are shown straight, instead of helically or spirally configured, for purpose of clarity; 
     FIG. 2 is a end view taken along lines  2 — 2  as shown in FIG. 1; 
     FIG. 3 is a representative cross-sectional view illustrating the barrel rifling actually present in FIG. 1; 
     FIG. 4 is a cross-sectional view taken along lines  4 — 4 , as shown in FIG. 1; 
     FIG. 5 is a cross-sectional view taken along lines  5 — 5 , as shown in FIG. 1; and 
     FIG. 6 is a partial view of a barrel depicting a variant of the transition section shown in FIG.  1  and in which the groove diameter is reduced. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a barrel  10  of a firearm, which may be a handgun, a rifle, or other ordinance. The barrel includes a chamber  12  representatively shown for receiving a cartridge supporting a projectile or bullet. The chamber is in communication with bore  14  of the barrel, which bore is essentially identical with the caliber of bullet  16  (see FIG.  3 ). A plurality of grooves  18  (opposed pairs or an odd number) are cut helically (sometimes referred to as spiral grooves) in the barrel, as shown in FIG. 3 to form the rifling in the barrel. Bottom surface  20  (see FIG. 2) of each of these grooves is essentially arcuate with a curvature as a function of the radial distance from longitudinal axis  22  of the bore and essentially define the bore of the barrel. As discussed above, the term groove diameter defines the distance between opposed pairs of grooves and the term groove radius defines the distance between the bottom surface of a groove and the axis of the bore. 
     Interleaved between adjacent grooves  18  are lands  30 . These lands (opposed pairs or an odd number) extend radially inwardly toward longitudinal axis  22  of the bore and are terminated by land surfaces  32  (see FIG.  2 ). The land surface may be flat or curved with a curvature being commensurate with the radial distance between the longitudinal axis of the bore and the land surface. The term land diameter, as discussed above, corresponds with the distance between opposed pairs of land surfaces and the term land radius defines the distance between the land surface and the longitudinal axis of the bore. 
     As particularly shown in FIG. 1, bore  14  includes essentially four sections. The first section adjacent chamber  12  is referred to as engraving section  40 . In the engraving section, the radial inward protrusion of lands  30  increases from a point  42  essentially coincident with the surface of the proximal end of bore  14  or distally therefrom to a point  44  of minimal land diameter distally therefrom. Normal section  50  extends distally from point  44  to a point  52  and is essentially of conventional rifling. Beginning at about point  52 , transition section  60  begins. In transition section  60 , the land diameter increases from point  52  to point  62  located distally therefrom. In terminal section  70  the land diameter is essentially constant to muzzle  80 . 
     When the firearm is discharged, bullet  16  being partially in engraving section  40  will travel therethrough and lands  30  will cut, form or engrave commensurately configured channels in the bullet. Simultaneously, the bullet will tend to expand radially due to the pressure of gases therebehind, inertia, and the resistance to forward movement caused by the rifling in the barrel. Such expansion will cause segments of the bullet to extend into grooves  18 , as illustrated in FIG.  4 . Although bullet  16  appears in FIG. 4 to be centered in the bore of barrel  10 , such is not usually the case, especially when engraving jacketed bullets. The above discussion is primarily pertinent to lead bullets. Jacketed bullets tend to become extruded longitudinally and may decrease somewhat in diameter when engraved. 
     A bullet is typically larger than the groove diameter but smaller than the free bore, if present. As the bullet enters engraving section  40 , some of the lands will cut, form or engrave the bullet to a deeper extent than other lands. This will result in the longitudinal axis of the bullet being off-center from longitudinal axis  22  of bore  14 . Since the longitudinal axis of the bullet passes through the center of gravity of the bullet, the center of gravity of the bullet will travel in a helical (spiral) pattern about the longitudinal axis of the bore. Such helical (spiral) travel creates a lateral velocity component to the forward motion of the bullet. Upon discharge of the bullet from muzzle  80 , the lateral component of force will act upon the bullet and affect its trajectory. As particularly shown in FIG. 4, bullet  16  is not laterally expanded sufficiently to ensure contact by the surface of the bullet with each of bottom surfaces  20  of grooves  18 . The amount of space therebetween will vary for each bullet and as between bullets discharged through the barrel. 
     Transition section  60  defines a section of increasing land diameter and therefore a reduction in radial distance between bottom surface  20  of each groove  18  and land surface  32  of each land  30 , the ratio of land diameter (radius) to groove diameter (radius) will increase. Such reduced difference urges bullet  16  to expand radially until each of grooves  20  is essentially filled and the bullet bears uniformly against each of bottom surfaces  20  (see FIG.  5 ). The resulting complete or essentially complete contact by the bullet with each of bottom surfaces  20  ensures that the longitudinal axis of the bullet, and hence its center of gravity, is coincident with longitudinal axis  22  of bore  14 . Any lateral velocity component that may have been previously present will become damped and no longer be present as the projectile/bullet passes from transition section  60  into terminal section  70 . 
     In terminal section  70 , the land diameters have been increased as a function of transition section  60  but remain constant to muzzle  80  to ensure complete expansion of the bullet into the grooves and damn any preexisting lateral velocity component upon discharge of the bullet. The resulting lack of lateral velocity component will permit the bullet to spin about its longitudinal axis and hence about its center of gravity as a result of the helical (spiral) rifling present in barrel  10 . 
     As discussed above, expansion of the surface of bullet  16  to fill grooves  18  can be accomplished by increasing the land diameter. A similar result can be achieved by reducing the groove diameter and thereby also reduce the difference between the land diameter and the groove diameter; or, increase the ratio of land diameter to groove diameter. Referring to FIG. 6, there is shown a variant of the bore construction shown in FIG.  1 . Transition section  60 A includes lands  30  with the land diameter being maintained essentially constant. The groove diameter is progressively decreased by reducing the depth of bottom surface  20  of each of grooves  18 . As bullet  16  passes into and through terminal section  70  it will have a configuration as depicted in FIG.  5  and discussed above in detail. Thereby, any lateral velocity component induced in the bullet is damped and eliminated by expansion of the bullet adjacent bottom surfaces  20  of grooves  18  with the resulting benefit of a spinning bullet being essentially unaffected by a lateral velocity component. 
     While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make the various modifications to the described embodiments of the invention without departing from the true spirit and scope of the invention. It is intended that all combinations of elements and steps which perform substantially the same function in substantially the same way to achieve the same result are within the scope of the invention.