This invention pertains to the making of ammunition cartridges for rifles such as those used by hunters, military, and competitive shooters.
Rifle chamber and ammunition cartridge designations are standardized by the Sporting Arms and Ammunition Manufacturer's Institute (SAAMI). ANSI maintains a corresponding standard Z299.4. These standards define, among other things, the physical dimensions of the cartridge and chamber for each cartridge designation by way of a mechanical drawing specifying the dimensions and tolerances for each feature. These dimensions and tolerances dictate how the cartridge will fit into the chamber, take into account changes in the cartridge dimensions during firing, and also account for normal manufacturing variation to ensure that all commercial ammunition will function in all commercial rifles. Allowable variations are small but they can have a significant effect on accuracy because they may alter the way a bullet enters the barrel, which effects how it leaves the barrel, which affects downrange accuracy. One of the motivations for hand loading ammunition is to take advantage of the ability to adjust the final dimensions of the cartridge to closely match the chamber of a particular rifle and to also decrease the variation from cartridge to cartridge thereby increasing accuracy and consistency.
Ammunition cartridges are assembled from a case, a primer, powder, and a bullet and are put into several broad classes based on the type of case that is used: rimmed, rimless, and belted being the most common types. Each of these three types of case use a different physical feature on the case to locate the case inside the chamber, which is commonly called ‘headspacing’ but within the SAAMI specification this is called ‘breeching’. Rimmed and belted cases are breeched by (“headspace off of” is the common terminology) the rim or belt, both features being located at the head of the case (end opposite the bullet). Rimless cases are breeched by (“headspace off of”) the shoulder, the conical transition between the larger cylindrical body of the case, which holds the powder, and the smaller cylindrical neck, which holds the bullet. This is a difference that the prior art has not addressed. All of the different reloading die, overall-length-gauges, bullet comparators, other tools, and the techniques for using them that are contained in the prior art and commercially available make no differentiation between these different case types, essentially treating all of them as if they were of the rimmed type where all critical dimensions are taken from the head of the case and alignment is controlled by adjustments to the body and neck of the case. However, for rimless shoulder breeching cartridges critical dimensions are properly referenced to the case shoulder and it is the case shoulder that should be used to align the bullet.
Among the issues that the existing technology is not handling correctly, for shoulder breeching cartridges, is the concentricity of the bullet with the bore of the barrel. The bullet is always slightly larger than the bore so the bullet is deformed as it is forced into and down the barrel, which forces the outer surface of the bullet into the rifling grooves, which causes the bullet to spin about the central axis of the barrel (not the central axis of the bullet). If the central axis of the bullet (which presumably goes through the center of mass) is not perfectly co-axial with the axis of the barrel then the bullet will be asymmetrically deformed and the center of mass will be forced to rotate about the axis of rotation. The result is a bullet which wobbles in flight, which has an unpredictable, and therefore deleterious, effect on downrange accuracy.
The problem with the existing tools and techniques appears to arise from a lack of appreciation of what the shoulder is doing in a shoulder breeching cartridge. For rimmed and belted cartridge cases the shoulder is just a transition between the enlarged body and the neck and it does not contact the shoulder of the chamber. Rimmed and belted cases rely on the interface of the rim and belt with the face of the rifle chamber to control the depth of the bullet and on the fitment of the body with the walls of the chamber to control concentricity. Therefore, in rimmed and belted cases the shoulder doesn't contribute to accuracy and can be ignored. Careful hand loaders, particularly competitive shooters using what is commonly called ‘bench rest’ loading techniques and tools (which do not take into account what the shoulder is doing) therefore spend a great deal of time worrying about aligning the exterior of the neck to the body in an effort to control concentricity of the bullet and the bore of the barrel. However, in a shoulder breeching cartridge the shoulder of the case is pushed against the shoulder of the chamber. This forced contact between case and chamber at the shoulder is what aligns the case and controls bullet depth. The body does not contact the chamber walls, so it is the body that should be ignored, not the shoulder.
This is a critical distinction that is being ignored by the existing technology so I have designed a set of tools for loading cartridges that are similar to existing tools but are designed specifically for shoulder breeching rifle cartridges. This includes tools for measuring the chamber and ammunition properly, a case hone, a neck sizing die, and a bullet seating die, each of which is the subject of a separate invention disclosure. This disclosure is for the neck sizing die.
This invention solves another weakness in existing designs that only resize the outer surface of the case neck. Any variation in wall thickness will cause the inner surface to be out of alignment and improperly sized even if the outer surface is perfectly aligned and sized. Because the inner surface of the neck is what positions the bullet, it is the concentricity of the inner surface with the bore of the barrel which affects accuracy. To resolve this tools have been developed that purport to ensure uniform wall thickness so that it can be presumed that alignment of the outer surface implies alignment of the inner surface. A better solution is to directly ensure that the interior surface is the correct diameter and properly aligned.