Abstract:
A cartridge for a firearm comprises a case having a base located at one end and a projectile mounted at the other end. A specific volume of propellant is contained in the case and is ignitable via a primer located in the base. The ignition of the propellant causes the projectile to be propelled from the case. The case comprises a wall defining a plurality of circumferential flutes that extend around outer and inner surfaces of the case in a helical or vertical configuration.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefits of U.S. Provisional Patent Application No. 61/175,923, filed on May 6, 2009, and U.S. Provisional Patent Application No. 61/230,855, filed on Aug. 3, 2009, the contents of both applications being incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to ammunition and, more particularly, to ammunition cartridges in which an outer surface of the cartridge is defined by flutes along at least a portion of the cartridge. 
     BACKGROUND 
     Standard ammunition cartridges for firearms are typically unitary in construction with the structural components of the cartridge being made from metal. In general, the cartridge includes a case of a generally cylindrical shape and terminated at a rearward end by a base having a rim. A propellant is contained in the case, and a primer is located in the base. A bullet or projectile is frictionally held in a forward end of the case. The case is sized to a particular caliber, which closely approximates the diameter of the projectile and is less than the diameter of the bore defined by the barrel of the firearm through which the projectile moves. When the cartridge is in battery located at a rearward end of the bore, operating the firearm causes the primer to be ignited (e.g., via a firing pin), which in turn ignites the propellant (usually gunpowder). Gases resulting from the ignition of the gunpowder result in an increase in pressure within the case, thereby causing the case to expand. Upon continued expansion of the case, the outer surface of the case seals against the wall of the firing chamber. Because the case cannot expand any further, there is a buildup of pressure in the case that causes ejecta to leave the case at its determined pressure so the projectile can achieve the correct velocity. The spent case is either removed manually or by the weapons operating system. 
     In commercial practice most ammunition is manufactured with drawn brass cases that are generally cylindrical and define a smooth outer circumferential surface that approximates the shape of the walls of the firing chamber. During firing of the cartridge, peak pressure is imparted to the case. The elasticity of the brass allows the case to expand diametrically under pressure and to contact the walls of the firing chamber forming a suitable seal in the firing chamber. In doing so, the engineered hoop strength of the material will not yield but will retain its original geometry through material memory. Once the pressure is relieved, the case returns to its original (or near original) condition. This quality, which is known as the “springback” of the case, facilitates the extraction of the case from the firing chamber. Without the case material exhibiting sufficient springback, the case would not return to its engineered taper, thereby resulting in increased friction at extraction and possibly malfunction. 
     SUMMARY 
     In one aspect, the present invention resides in a cartridge for a firearm. The cartridge comprises a case having a base located at one end and a projectile mounted at the other end. A specific volume of propellant is contained in the case and is ignitable via a primer located in the base. The ignition of the propellant causes the projectile to be propelled from the case. The case comprises a wall defining a plurality of circumferential flutes that extend around outer and inner surfaces of the case in a helical or vertical configuration. 
     In another aspect, the present invention resides in a cartridge for a firearm. The cartridge comprises a case having a wall arranged to define a substantially cylindrical member having a forward end, a rearward end, and inner and outer surfaces, a projectile located at the forward end of the case, and a base located at the rearward end of the case. A specific volume of propellant is located in the case and is in communication with and configured to be ignited by a primer located in the base through a flash hole. Each of the inner surface and the outer surface of the case defines a plurality of flutes that extend helically or vertically along the substantially cylindrical member. 
     In another aspect, the present invention resides in an assembly for an ammunition cartridge. This assembly comprises a substantially cylindrical case and a base located at a rearward end of the case. The case, which is fabricated from a partial polymeric material, comprises a wall configured to define a plurality of flutes extending longitudinally between the rearward end of the case and a forward end of the case with the plurality of flutes being defined on inner and outer surfaces of the wall. The base comprises a metallic insert that houses the primer and further creates a metallic rim for ejection from the weapon, its upper portion creates a new feature or flash base and also the traditional flash hole. The base also includes a body, the body being formed from the partial polymeric material and over-molded on at least a portion of the housing. The body further defines an outer surface having a plurality of flutes that matingly engage the flutes defined by the inner surface of the case in a close fit to allow for bonding adhesive to be inserted at time of assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a cartridge, of the present invention. 
         FIG. 2  is a side view of the cartridge of  FIG. 1 . 
         FIG. 3  is a side sectional view of the cartridge of  FIG. 1 . 
         FIG. 4  is another side sectional view of the cartridge of  FIG. 1 . 
         FIG. 5  is a side sectional view of an area in a neck of a case of the cartridge of  FIG. 1 . 
         FIG. 6  is another side sectional view of the area in the neck of the case of the cartridge of  FIG. 1 . 
         FIG. 7  is a side view of a cartridge of the present invention. 
         FIG. 8  is a top sectional view from a forward end of the case. 
         FIG. 9  is a bottom sectional view from a rearward end of the case. 
         FIG. 10  is an exploded view of the cartridge, of the present invention, compared to a prior art cartridge. 
         FIG. 11  is a perspective view of a base of the cartridge of  FIG. 1 . 
         FIG. 12  is a perspective view of a base of the cartridge in which a body is over-molded onto a housing. 
         FIG. 13  is a cutaway perspective view of the base of  FIG. 12 . 
         FIG. 14  is a perspective view of the housing of the base of  FIG. 12 . 
         FIG. 15  is a perspective view of the cartridge, of the present invention, shown in phantom. 
         FIG. 16  is a side view of a physical model of the cartridge, of the present invention, compared to a prior art cartridge. 
         FIG. 17  is a top view of the case of the cartridge of  FIG. 1 . 
         FIG. 18  is a top view of the case of  FIG. 17  in which the case is filled with propellant. 
         FIG. 19  is a perspective view of a determined amount of propellant being weighed for use in a cartridge, of the present invention. 
         FIG. 20  is a side view of the case of the cartridge, of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1-4 , a cartridge for use in a firearm is shown generally at  10  and comprises a case body defined by a case  12 , a propellant contained in the case, a base  14  that is inserted into the case body, and a projectile  16  mounted in the case. The primary use for the cartridge  10  of the present invention is with regard to small arms ammunition such as 5.56 mm (NATO) ammunition and larger through 50 BMG (Browning Machine Gun) ammunition. The present invention is not limited in this regard, however, as other sizes of ammunition can employ the configurations disclosed herein, particularly with regard to pistol, rifle, and grenade case (30 mm and 40 mm) ammunition. In any embodiment, however, the case  12  is substantially cylindrical in shape and defined by a wall  18 . The wall  18  defines an interior area of the case  12  that contains the propellant. The base  14  is located on a rearward end of the case body and includes a primer for igniting the propellant when the cartridge  10  is fired. A forward end of the case body includes a shoulder portion  20  that tapers into a neck portion  22 . The projectile  16  is mounted in the neck portion  22 . 
     The case  12  and at least portions of the base  14  may be fabricated from one or more polymeric materials. The polymeric material may be a composite defined by a polymer or polymeric matrix that contains one or more of glass fiber, carbon fiber, carbon nanotubes, and combinations of the foregoing materials. Another polymeric material found to be suitable as a material for the case  12  is polyetheretherketone (PEEK) functionalized with 2-5 wt.% of carbon nanotubes. Additives may be incorporated into the polymeric material, such additives including, but not limited to, wetting agents, molding agents, release agents, colorants, combinations of the foregoing, and the like. The present invention is not limited to the polymeric material being a composite or PEEK, however, as other materials such as polyetherketone (PEK), polyphenylsulfone, combinations of the foregoing materials, and the like may be used. 
     Referring now to  FIGS. 5 and 6 , the wall  18  of the case  12  is thicker in the area of the shoulder portion  20  and the neck portion  22  than it is rearward of the shoulder portion, thereby reinforcing the neck portion to ensure that the projectile  16  can be suitably mounted in the case  12  in a mechanical interference fit and retained in the cartridge  10 . The thicker portion of the wall  18  also ensures that, upon firing, pressure in the cartridge is contained until the pressure reaches a desired level whereupon the projectile  16  is caused to separate from the case  12 . 
     As can be seen in  FIG. 7 , the wall  18  defines a plurality of flutes  30  on the outer surface of the case  12  having a centerline C. The flutes  30  extend along the length of the outer surface of the case from the rearward end to the forward end and terminate proximate the shoulder portion  20 . In the illustrated embodiment, the flutes  30  are helically arranged around the case  12  at an angle of about 2 degrees to about 20 degrees. However, the present invention is not limited in this regard as the flutes can also be straight or be configured in other patterns without departing from the broader aspects of the invention. 
     The helical arrangement of the flutes  30  on the outer surface of the case  12  forms a corresponding helical arrangement of the flutes  30  on the inside of the case  12 . On the inside of the case  12 , however, the flutes  30  extend through the shoulder portion  20  and to the neck portion  22 . The helical arrangement of the flutes  30  on the inside of the case  12  allows the base  14  to be matingly attached to the case body in a mechanical interference fit after which the base is glued or comelted to the case body. The case body is a separate component that is molded, extruded, machined, or otherwise formed and to which the base  14  and the projectile  16  can be attached. 
     Referring now to  FIGS. 2 ,  8 , and  9 , the wall  18  of the case  12  defines a plurality of flutes  30  on an inner surface  32  of the wall ( FIG. 8 ). Flutes  30  are also defined by an outer surface  34  of the wall  18  ( FIG. 9 ). The distance between a peak of a flute  30  on the outer surface  34  and a peak of a flute on the inner surface  32 , as well as the distance from either peak to an adjacent peak, is calculated to provide a helical arrangement of the flutes  30  having a desired configuration, thereby imparting predetermined mechanical properties to the case  12 . The helical arrangement of the flutes  30  is selected to improve the strength of the case  12  (relative to cases of the related art) in a cross-sectional direction (the “hoop strength”) and also enhances the compressive loading (force exerted on the case along the centerline C), thereby allowing the case to flex to accommodate the insertion of the projectile  16  into the neck portion  22 . The helically-arranged flutes  30  can be configured to either minimize the potential for cartridges to interlock from one case to the next or to enhance the belt feeding of cartridges by creating latching surfaces on the outer surfaces  34  of the cases  12 . Furthermore, the flutes  30  provide for a reduction in the surface area of the case  12  (relative to straight wall cases of the related art) that contacts the walls of the firing chamber, thereby reducing the amount of heat transferred from the walls of the firing chamber to the case and inhibiting the softening or melting of the polymer. Reducing the amount of heat transferred from the walls of the firing chamber to the case  12  may also reduce the potential for cook off. Additionally, by manufacturing the case  12  from the polymer (at least in part) instead of brass or other metal, the weight of the case is reduced, thereby also reducing the weight of the cartridge. For example, in a 50 BMG cartridge, the overall weight of the case is reduced by about 47% and the overall weight of the cartridge is reduced by about 15% (as compared to a similar cartridge incorporating brass instead of polymer). 
     In addition to improving the hoop strength, reducing the heat transfer abilities, and reducing the weight of the cartridge, the helical arrangement of the flutes  30  reduces the amount of friction in the extraction of the spent case  12  from the firing chamber. In particular, the flutes  30  reduce the amount of contact the case  12  has with the walls of the firing chamber such that when the spent case is engaged by an extraction device and pulled in a rearward direction for ejection from the firearm, the amount of heat generated from the friction due to extracting the spent case is minimal (reduced by about 70%). Furthermore, the portion of the case  12  in the area of the base  14  along the edge at which the flutes  30  terminate is strengthened by the flutes  30 , thereby resisting substantial deflection of the wall of the case  12  during the process of extracting the case from the firing chamber and ejecting the case from the firearm. 
     Also, the flutes  30  can be helically arranged at the desired angle accordance with the rotational movements of the cartridge  10  in the firearm. For example, when the firearm is a rifle having a 1:4 twist, the helical arrangement of the flutes  30  on the case  12  of the cartridge  10  for the rifle can have a corresponding degree of spiral around the case such that the twist defined by the flutes on the case matches the twist in the bore of the rifle. In doing so, the ballistic qualities of the cartridge  10  can be improved over the cartridges of the related art, particularly cartridges having cases defined by non-fluted walls. 
     Referring now to  FIG. 10 , at least a portion of the base  14  is also substantially cylindrical in shape and includes a wall that is fluted on the outside. The flutes  40  are helical and positioned similarly to the flutes  30  defined by the inside surface  32  of the wall  18 , thereby allowing the base to mate with the case. As can be seen, the cartridge  10  is similar in size and shape (except for the flutes  30  on the case  12 ) to a typical cartridge  42 , which in this case is a 50 BMG cartridge. 
     Referring now to  FIG. 11 , the base  14  includes a rim  44  at a rearward end of the substantially cylindrical portion. The rim  44  includes a relief or channel  46  extending circumferentially therearound to allow a suitable mechanism to engage a rearward surface  48  defining the channel  46  (in the process of extracting a spent cartridge  10  after firing and ejecting the cartridge). A hole  50  extends through a bottom surface  52  of the base  14  to provide communication between a primer located in the bottom surface and the propellant carried by the cartridge  10 . 
     The base  14  (and the rim  44 ) can be manufactured by any suitable operation. In one operation, the base  14  can be manufactured in a stamping process (particularly if the base is made at least in part of a metal such as aluminum). 
     In another operation, the base  14  as shown in  FIGS. 12-14  can be manufactured using an insert molding process. The base  14  manufactured using the insert molding process comprises a stamped housing  82  over which a body  84  is molded. The over-molded material of the body  84  is preferably the same material as is used for the case body. Utilizing the same materials for the body  84  and the base allows the case body to be received in the base and joined thereto in a comelt or glued bond. One or more acetyl or cyanic-based adhesives can be employed to join the case body  84  of the base  14  to the case. 
     Referring to  FIG. 14 , the housing  82  is preferably steel, although other materials may be used. Using steel (or at least another metal or alloy) allows for efficient extraction of cases by enabling an ejector to engage an upturned edge of the rim  44  (in the process of extracting a spent cartridge from the firing chamber after firing), thereby allowing for extraction and avoiding subjecting the polymeric material of the case  12  directly to the forces of the extraction which may compromise the integrity of the case. The housing  82 , as shown in  FIG. 14 , includes the rim  44  and a rearward surface  84  that defines a rearward end of the substantially cylindrical portion of the case into which the base  14  is inserted. The hole  50  extends through the base  14  from the rearward surface  84  to a forward surface  88 . A primer can be located in the hole  50  in any suitable manner (e.g., by being press fit or by using staked insertion). 
     The forward surface  88  of the base defines a cone or flash pan with the inside concave portion thereof facing forward. An angle  90  defined by the forward surface  88  relative to a plane P perpendicular to the centerline C extending longitudinally through the case  12  is about 10 degrees. The present invention is not limited in this regard, as the angle  90  may be more or less than 10 degrees. By configuring the concave portion of the forward surface  88  to have an angle of about 10 degrees, however, faster ignition of propellant, as compared to the forward surface being flat, can be realized. More specifically, upon ignition of the primer in the hole  50 , the propellant proximate the rearward end of the case  12  is ignited first, and the ignition is propagated through the propellant to the forward end of the case. By angling the forward surface  88 , the ignition can be directed to the forward end of the case, thereby limiting the amount of early ignition of the propellant in the lateral directions (e.g., perpendicular to the centerline C). Furthermore, the helical arrangement of the flutes  30  may further contribute to the propagation of the ignition from the rearward end to the forward end by directing the ignition along the walls of the case  12  in the flutes  30 . 
     As shown in  FIG. 15 , upon insertion of the base  14  into the rearward end of the case body, the flutes  40  are received in the flutes  30  defined on the inside surface  32  of the wall  18  of the case  12  in the interference fit and joined in a comelt or glued bond. One benefit of incorporating an insertable base  14  having flutes  40  that are received in the case  12  in a mechanical interference fit and joined in a comelt or glued bond is that the amount of surface area usable for engaging and bonding the base to the case is increased. The increase in engaging and bonding surface area provided by the flutes  30  on the case  12  provides a bond that is significantly greater than the bond effected in similar case/base assemblies having smooth engaging walls. More specifically, with regard to cartridges  10  for small arms as described herein, the increase in the usable surface area for engaging and bonding the base to the case is about 55% (as compared to non-fluted cartridges  10 ). 
     In joining the base  14  to the case  12  as described herein, another benefit is realized in that the mechanical interference joint (with the comelt or glued bond) does not experience the full pressure of the ignition of the propellant. Due to the twist of the helical arrangement of the flutes  30  of the case  12  engaged with the flutes  40  of the base  14 , about 30% of the force in the rearward direction from the ignition of the propellant is mitigated due to the mechanical joint created by the helical relationship. In doing so, only about 70% of the pressure is experienced by the base  14  in a direction parallel to the centerline C. Thus, the helical arrangement of the flutes contributes to the mechanical joining of the base  14  to the case  12 . 
     Referring now to  FIG. 16 , the cartridge  10  can be designed using rapid prototyping (RPT) techniques. These RPT techniques take virtual designs from computer aided design or animation modeling software, transform the designs into virtual cross-sections, and then create each cross-section in physical space using an RPT material, assembling the cross-sections to define a physical model  60  that corresponds to the virtual designs. As can be seen in a comparison  100 , the physical model  60  that is used in the development of the cartridge  10  is a close approximation of a typical 50 BMG cartridge  42 . The desired elevation (height of the cartridge  10  from the base to the forward-most end of the projectile  16 ) is determined by the overlap of a bond area  62  (the area at which the neck of the case  12  and the projectile overlap in an assembled cartridge). The present invention is not limited to 50 BMG cartridges, however, as any other cartridge caliber is within the scope of this disclosure. 
     In the present invention, the characteristics of the RPT material (e.g., density) used to fabricate the physical model  60  closely approximate the characteristics of the polymer used to fabricate the case  12  of the cartridge  10 . This allows for actual measurement data to be obtained in instances where data cannot be calculated. For example, using the physical model  60 , actual data can be measured for charge weights and volumes (amount of propellant), actual weight savings per round, measurement of surface areas at which the case engages the wall of the firing chamber, and measurement of surface areas at which various portions of the cartridge  10  are bonded or otherwise attached to each other. Also, visualization of prospective or actual processes of manufacture (such as molding) can be carried out using the physical model  60 . 
     The embodiments of the cartridge  10  described herein and its methods of manufacture can be used with traditional ammunition manufacturing equipment (such as a SCAMP line). In particular, a molded (or otherwise formed) case and base can be built as subcomponents and assembled. In one method of assembly, a base  14  can be attached to a case  12 , propellant charged to the case, and a projectile  16  fitted to the case. In another method of assembly, the projectile  16  can be attached to the case  12 , the case charged with propellant, and the base  14  attached to the case. The adaptability of toggling between such methods provides the cartridge  10  of the present invention with several advantages. 
     One advantage of subcomponent manufacturing is that at least some of the subcomponents manufactured are inert. Different subcomponents can be provided by different manufacturers, at different facilities, or by the same manufacturer at different facilities or locations. Thus, the level of security afforded to the manufacture of ammunition can be varied depending on the particular subcomponent. Furthermore, just-in-time (JIT) techniques can be used in the assembly of the subcomponents, which means that a multitude of manufacturers can be employed, thereby eliminating the need for stand-alone munitions plants. 
     Another advantage is that costs associated with demilling live ammunition can be mitigated. Because polymers are used in the present invention, and further because the cartridges of the present invention can be manufactured as subcomponents and assembled, the various subcomponents can be destroyed or recycled on an as-needed basis. Because of this subcomponent manufacturing and the capability for JIT assembly, it has been discovered that demilling costs on the order of about 50% can be saved by making fewer finished cartridges (live ammunition) and stockpiling fewer subcomponents. 
     Example 1 
     Propellant Charge Weight Evaluation 
     The physical model  60  ( FIG. 16 ) was manufactured with the projectile at the desired location in the case from animation modeling software in accordance with government specifications. The cartridge  10  was then developed based on the physical model  60 . Using the animation modeling software to manufacture the physical model  60  and developing the cartridge  10  from the physical model enabled accurate propellant charge weight measurements to be obtained. A 50 BMG cartridge made of brass was determined to weigh 0.284 pounds (lbs.), and a cartridge  10  of the present invention was determined to weigh 0.193 lbs. The weight reduction was 0.091 lbs. 
     In the cartridge  10 , referring now to  FIGS. 17 and 18 , the case  12  (empty in  FIG. 17 ) was then filled to the desired level with propellant  70  ( FIG. 18 ) and weighed to determine the amount of propellant charged. 
     In some embodiments, a charge bag (e.g., a pouch or envelope) was inserted into the case  12  before filling with propellant  70 . The charge bag shaped the propellant charge to correspond with the case  12  in the area of the base  14 . In some embodiments, the charge bag left multiple air channels in the voids of the propellant charge, these air channels providing for accelerated ignition of the cartridge  10  upon firing and thereby yielding a higher projectile velocity. The charge bag could be conical in shape to allow the base  14  to have the needed egress for assembly, thereby allowing additional grains of propellant to be housed in the base of the cartridge  10  above the primer. 
     Referring now to  FIG. 19 , the propellant  70  charged to the case  12  was in accordance with government specifications. 
     Example 2 
     Cartridge Weight Evaluation 
     Referring now to  FIG. 20 , the physical model  60  ( FIG. 16 ) enabled an accurate weight measurement of a manufactured cartridge  10  to be taken, which allowed further computations to be made. The cartridge  10  produced from the physical model  60  was sufficiently translucent to enable the propellant  70  located in the case  12  to be observed. Furthermore, the translucency enabled the bond area  62  to be discerned. 
     Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the appended claims.