Abstract:
The freebore section of a barrel of a firearm includes a plurality of freebore pads interleaved with a plurality of freebore slots for conveying propellant gases past the bullet upon entry of the bullet into the freebore section to stabilize and maintain the bullet in axial alignment with the axis of the rifled bore of the barrel upon entry of the bullet into the rifled bore.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application includes subject matter contained in and claims priority to the subject matter disclosed in a provisional application entitled “FIREARM FREEBORE STRUCTURE” filed Sep. 15, 1999 and assigned Ser. No. 60/154,108 directed to an invention made by the present inventor. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to firearm barrel construction and, more particularly, to the structure of the freebore in the barrel of a firearm.  
           [0004]    2. Description of Related Art  
           [0005]    When a round of a firearm is chambered, the projectile or a bullet is located adjacent the freebore of the barrel, which freebore is a space in the barrel downstream of the chamber and prior to the beginning of rifling in the barrel. Primarily, the freebore serves in the manner of a guide to introduce the bullet to the rifling. Once the bullet enters the rifled portion of the barrel, the lands and groves of the rifling engrave the bullet and introduce a spinning motion to the bullet about its longitudinal axis to stabilize the bullet along its path after it exists the muzzle. Ideally, the bullet is in axial alignment with the rifling to cause the engraving of the bullet to be uniform about its longitudinal surface. Furthermore, ideally, the gas pressures, upon firing of the propellant within the cartridge, will be uniformly dispersed behind and about the bullet as the bullet enters the rifled section of the barrel. Unfortunately, neither is always true and consecutively fired bullets will have somewhat different trajectories as a result of non-symmetrical engravings and non-uniform lateral gas pressures. Depending upon the purpose for which the firearm is used and the distance of the trajectory, the resulting variations of trajectory may or may not be within satisfactory limits.  
         SUMMARY OF THE INVENTION  
         [0006]    A plurality of circumferentially placed slots extend axially along the freebore section of a rifled barrel to equalize the lateral gas pressures acting upon the bullet and prevent occurrence of lateral pressure differentials which would misalign the longitudinal axis of bullet relative to the longitudinal axis of the barrel. Moreover, the forwardly escaping gases around the circumferential surface of the bullet will tend to assist in compressing the bullet radially inwardly to assist engraving of the bullet upon contact with the radially converging lands of the rifling of the barrel.  
           [0007]    It is therefore a primary object of the present invention to maintain the longitudinal axis of a bullet coincident with the longitudinal axis of a rifled barrel upon discharge of the propellant.  
           [0008]    Another object of the present invention is to provide a plurality of circumferentially spaced slots in the freebore section of a rifled barrel to provide uniformly pressurized gas flow past a bullet in the freebore section.  
           [0009]    Still another object of the present invention is to provide a freebore section of a rifled barrel which will tend to axially align and axially maintain a bullet as it enters the rifled section of the barrel.  
           [0010]    Yet another object of the present invention is to provide channeling within a freebore section of a barrel to minimize pressure differentials about the circumferential surface of a bullet prior to entry of the bullet into the rifling of a barrel.  
           [0011]    A further object of the present invention is to provide a source of uniform pressures about the front of a bullet to assist in engraving the bullet as it enters the rifling of a barrel.  
           [0012]    A still further object of the present invention is to provide uniformly spaced axial slots in the freebore section of the barrel of a pistol to increase the accuracy and repeatability of bullets fired from such a pistol.  
           [0013]    A yet further object of the present invention is to provide a method for improving the accuracy of the rifled barrel of a firearm.  
           [0014]    These and other objects of the present invention will become apparent to those skilled in the art as the description thereof proceeds.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The present invention will be described with greater specificity and clarity with reference to the following drawings, in which:  
         [0016]    [0016]FIG. 1 illustrates a partial cross section of the barrel of a firearm showing a segment of the chamber, the freebore section and a segment of the rifled section;  
         [0017]    [0017]FIG. 1A illustrates a wall at the end of a freebore slot; and  
         [0018]    [0018]FIG. 1B illustrates a ramp at the end of a freebore slot;  
         [0019]    [0019]FIG. 2 is a cross sectional view taken along the lines of  2 - 2 , as shown in FIG. 1;  
         [0020]    [0020]FIG. 3 illustrates a grouping of five (5) rounds of each of five (5) different brands of commercially available ammunition;  
         [0021]    [0021]FIGS. 4A, 4B,  4 C,  4 D,  4 E and  4 F illustrate the grouping of five (5) rounds obtained from each of six (6) different brands of commercially available ammunition; and  
         [0022]    [0022]FIG. 5 illustrates a typical 45 caliber semi-automatic pistol in which the barrel of the present invention may be used.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    When a cartridge is fired in a firearm, the pressure of the gaseous propellant in the case acts on the base of the cartridge attached bullet forcing it forwardly out of the cartridge. The bore of the firearm includes a short freebore section leading to a converging portion of the bore wherein the bullet begins to become engraved by the rifling. The friction on the lands of the rifling on one side of the bullet can be greater than the friction on the opposite side of the bullet which can cause the bullet to become somewhat cocked to one side; that is, the longitudinal axis of the bullet becomes misaligned with the longitudinal axis of the bore. Such initial cocking of the bullet has an effect upon the trajectory of the bullet after it leaves the muzzle of the firearm because the amount or degree of cocking upon successively discharged bullets is not necessarily the same. Highly accurate repeatability of the trajectory of successively fired bullets is compromised. Thus, the accuracy of the firearm is compromised.  
         [0024]    The propellant gases moving forwardly along a side of a bullet after separation of the bullet from the cartridge could, because of the presence of restricted gas flow or mechanical asymmetries, momentarily cause somewhat more pressure to be present on one side of the bullet then on the other side within the freebore section of the barrel. The bullet will respond to such pressure differentials by moving laterally away from the high pressure side resulting in an increasing gap on the high pressure side. The increasing gap increases propellant gas flow therethrough and the reduced gap on the opposite side will restrict propellant flow therethrough. Such gas pressure differentials will urge the bullet to one side of the freebore. As the tip of the bullet enters the converging portion of the bore the tip will be urged into alignment with the longitudinal axis of the bore and result in misalignment of the longitudinal axis of the bullet with respect to the longitudinal axis of the bore and produce a cocking of the bullet.  
         [0025]    As the laterally displaced and/or cocked bullet moves forwardly from the chamber into the freebore section, it will contact the relatively sharp corner adjacent the freebore section of the barrel. Such contact will form a gas seal and prevent propellant gas from passing forwardly along the contacted side of the bullet. Because of such gas flow obstruction, the pressure of the gas adjacent the bullet forwardly of the point of contact will be relatively low compared to the pressure of the gases on the opposite side of the bullet. Such resulting gas pressure differential may have the effect of the deforming the bullet to some extent. The resulting deformation will affect the trajectory of the bullet after it leaves the muzzle. Furthermore, even if the pressure differential does not deform the bullet, the non-uniform pressures acting on the bullet may cause the traversed relatively sharp corner defining entrance to the freebore section to deform the bullet as it slides there across.  
         [0026]    Irrespective of how the bullet may be cocked at the point of engaging the rifling, the resulting engraving of the bullet will be nonuniform and the bullet will be caused to spin about an axis which is not coincident with the longitudinal axis of the bullet. The resulting distortion of the geometry of the bullet would have a negative effect upon repeatability of the trajectory of the bullet and hence the accuracy of the firearm.  
         [0027]    Even if one ignores the potential problems resulting from the bullet being off center or cocked as it enters the rifling in the bore of a barrel, another potential problem exists. Upon discharge of the propellant in the cartridge, the base of the bullet experiences fall chamber pressure. The relatively small gaps between the side of the bullet and the inside wall of the cartridge neck retaining the bullet and between the side of the bullet and the interior surface of the freebore section would restrict the flow of propellant gas forwardly. This can result in a relatively low pressure on the circumferential surface of the bullet compared to the pressure acting upon its base. This pressure differential has a potential of extruding the bullet outwardly until the outside of the bullet yields outwardly until constrained by the neck section of the cartridge and/or the interior surface of the freebore section.  
         [0028]    Eventually, the bullet is stuffed into the rifled section of the bore due to the forces of the propellant gas and substantially assumes the cross section of the bore. The above described possible forms a bullet misalignment, deformation and distortion prior to entering the rifled section of the bore along with the further deformation caused by the rifling renders it unlikely that the bullet is returned to its manufactured configuration or that its longitudinal axis is coincident with the and/or longitudinal axis of the bore. As discussed above, the highly likely misalignment and/or bullet distortion will result in a reduction in accuracy.  
         [0029]    Referring to FIG. 1, there is shown a barrel  10  of a firearm defining a bore  12 . The bore includes conventional rifling formed by lands  14  interweaved with grooves  16 . Although the rifling is shown straight for illustrative purposes, it is to be understood that the rifling defines a helix (spiral) to impart rotation to a bullet traversing therealong. The barrel includes a chamber  20 . A freebore section  22  extends toward the muzzle from the chamber to a conical converging section  24 . The conical converging section terminates at about end  26  of tapered rifling lands  28 . A plurality of slots  40  are disposed in freebore section  22  open to chamber  20  and extend toward the muzzle more or less parallel with longitudinal axis  30  of the barrel toward conical converging section  24 . The ends of slots  40  may include a wall  32  perpendicular to longitudinal axis  30 , as shown in FIG. 1A. Alternatively, the ends of the slots may be defined by a ramp  34 , as shown in FIG. 1B.  
         [0030]    Slots  40  serve in the nature of passageways to permit propellant gases to move forwardly into the length of the freebore section relatively unimpeded. Thereby, the slots allow the base and circumferential surface of the bullet to experience a substantially uniform compressive pressure. Therefore, the unengraved portion of the bullet will not experience any of the pressure induced distortions discussed above. The resulting compressive gaseous pressure field also facilitates extruding the bullet into the rifled bore. That is, the external pressure leaking forwardly along the bullet will squeeze or compress the bullet radially inwardly and diminish somewhat the force required to have the rifling engrave the bullet.  
         [0031]    Freebore lands or pads  42  (see FIG. 2), defining the original freebore surface, can serve to guide the bullet and keep it substantially centered as it moves forwardly toward the muzzle to become engraved. As the bullet travels into converging conical section  24  it forms a seal with the bore at the location wherein the diameter of the converging conical section matches the diameter of the bullet. Subsequent forward movement of the bullet impresses rifling lands  14  into the side of the bullet to engrave the bullet.  
         [0032]    In one experiment of the present invention, the diameter of freebore slots  40  was equal to the diameter of chamber  20 , as illustrated. However, the diameter could be more or less than the diameter of the chamber. While the number of freebore slots are depicted as being equal in number to and in general alignment with rifling lands  14 , the number of freebore slots could be more or less. Alternatively, the freebore slots may be in general alignment with rifling grooves  16  and of greater or lesser number than the number of rifling grooves. The widths of freebore slots  40  may be essentially equal to the width of grooves  16  of the rifling. Narrower freebore pads  42  would provide less impediment to uniform pressure distribution about the bullet but the freebore pads would have to have sufficient surface area to support the bullet and keep the bullet centered without producing deformation of the bullet surface. There may be erosion of the freebore pad surface by the propellant gases and such potential erosion may have an effect upon the preferable minimum width of the freebore pads. Forward wall  32  of freebore slots  40  may be perpendicular to the longitudinal axis of the bore, as depicted in FIG. 1A but it may be preferable to incline these forward wall in the form of a ramp  34  (see FIG. 1B) to provide a more shallow angle between the bottom surfaces of the slots and the lands of the rifling. Conventionally, the length of the freebore section is approximately equivalent with the radius of the bullet. Whether such length is optimum its presently unknown pending further experimentation.  
         [0033]    Based upon a present understanding of the invention, it is preferable that the radial distance from the longitudinal axis  30  to the freebore pad be slightly larger than the radius of the bullet to allow for some clearance for the bullet. Conical converging section  24  preferably extends radially inwardly from the freebore pad radius to the radially converged section of tapering lands  26 . As a variant, freebore slots  40  could be extended further into conical converging section  24 . However, it is expected that the forward limit of freebore slots  40  would preferably be aft of the location at which the diameter of conical converging section  24  equals the diameter of the bullet; that is, the freebore slots would terminate just short of the point at which the bullet would seal the bore.  
         [0034]    To test the effect of the modifications to the barrel shown in FIGS. 1 and 2 and described above, the barrel of a 0.45 caliber semi-automatic pistol was modified to incorporate such a barrel. The pistol was mounted in a test stand and fired at a target 20 yards away. Numerous manufacturers provide ammunition for 0.45 caliber semi-automatic pistols. Depending upon quality control standards and other variables present during manufacture of such ammunition, some brands are historically more consistent and therefore more accurate then other brands. To provide a demanding test of the effect on accuracy of the above described modifications to a barrel, 5 brands of ammunition which typically exhibit poor accuracy were selected. Five (5) rounds of each such brand of ammunition were fired through the barrel and produced the pattern of 25 shots illustrated in FIG. 3. By comparison, this pattern is significantly tighter and more accurate than any pattern produced by any one of these brands of ammunition fired through a standard, unmodified barrel.  
         [0035]    To further test the accuracy of the barrel as a result of the modifications shown in FIGS. 1 and 2, 5 rounds of 6 different brands of commercially available ammunition were fired through the barrel. The resulting 6 patterns are illustrated in FIGS.  4 A- 4 F. Again, these patterns are extremely tight compared to the patterns obtained by the same brands of commercially available ammunition fired through conventional unmodified barrels. One must therefore come to the inescapable conclusion that control of the propellant gases to preclude misalignment, distortion and/or deformation of the bullet as it traverses the freebore section has a significant effect upon the accuracy and such accuracy is available from a barrel modified as described above and illustrated in FIGS. 1 and 2.  
         [0036]    A typical handgun  50  in which the above described barrel may be incorporated is illustrated in FIG. 5. The original version of this pistol was made by the Springfield Armory in 1911 for use by the army. Its formal designation is 1911-A1 pistol firing 0.45 caliber bullets. It is to be understood that the barrels of other types of handguns and rifles may be modified to incorporate the substance of the present invention and thereby become more accurate firearms.  
         [0037]    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.