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 affects 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 cases 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 dies, 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 referenced from the head of the case and alignment is controlled by adjustments to the body and neck of the case. For 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 other things, careful hand loaders are concerned with the distance that the bullet moves before it engages the rifling of the barrel (called ‘bullet jump’) and the concentricity of the bullet with the bore of the barrel. Several problems arise when loading shoulder breeching cartridges with existing die designs. One is that variations in the length of the case body between the head and the shoulder are translated into variations in bullet jump. This does not occur with rimmed or belted cases because the rifle chamber and the loading press/die are using the same control point, the rim or belt. Existing die and presses are designed to control the Cartridge Overall Length (C.O.L.), the length from the head of the case to the tip of the bullet. This is the sum of two values: the length of the case body from the head of the case to the shoulder and the length from the shoulder to the bullet tip. With existing die designs, a reduction in the length of the case body will result in an increase in the distance from the shoulder to the bullet tip but the C.O.L will remain unchanged. For rimmed and belted cases it is changes in C.O.L. that cause changes in bullet jump so these case designs are not affected by changes in body length. But in a shoulder breeching case, it's changes in the distance from the shoulder to the bullet that cause changes in bullet jump. Therefore, with present die designs any change in the case body length will cause a change in the final shoulder to bullet length, which will cause a change in bullet jump. Because variation in body length from case to case naturally exists, uncontrolled variation in bullet jump must be the result with existing designs.
Another problem that arises from using existing die designs with shoulder breeching cartridges is that changes in bullet shape also affect bullet jump. Even within a box of the same bullets from a reputable maker significant amounts of variation are expected, so many loaders weigh each bullet to create batches that all have the same weight. Any change in weight necessarily translates into changes in length at or near the tip, so even when using the same bullets from the same box there is variation in length that will cause changes in bullet jump. Additionally, hand loaders use a variety of bullets in the same case that have significant differences in the shape of the tip. A longer bullet will cause the bullet to be pushed farther into the case, which increases bullet jump. Therefore, a hand loader wishing to control bullet jump needs to carefully adjust existing seating die designs every time they use a different bullet type.
Existing seating die designs also handle concentricity incorrectly for shoulder breeching cases. A bullet is deformed by the rifling grooves as it is forced into and down the barrel and those grooves force 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 is not perfectly concentric to 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. This 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 case. 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 (which positions the bullet) 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 and is the only point of contact between the case and 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. These include 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 bullet seating die.
I will be using the term ‘datum line’ in the description of the invention. This is a term I have found used in some of the relevant literature and it appears to have been in use and well accepted for a long time and thus reasonable to adopt. However, the only official documentation is the SAAMI specification, which does not use this term, so it needs to be defined. The ‘datum line’ is not a visible feature on the cartridge but is the median diameter of the conical shoulder. In the SAAMI Cartridge and Chamber Drawings for shoulder breeching cartridges the datum line is the line drawn through the center of the shoulder, denoted with a “B” for being a ‘basic dimension,’ and also denoted with an ‘x’ inside an ‘o’ on the chamber drawing for being the ‘headspace dimension’. This is also the only place in the drawings where the dimension for the cartridge and the matching dimension for the chamber are the same value. This means that the shoulder is the only place where the cartridge is expected to make contact with the chamber. For an example, see the drawings for the 30-06 Springfield and find the line through the shoulder with a diameter of 0.375 inches.
I need to create a new term: ‘bullet datum’, which I define as the place where the cross sectional diameter of the bullet is the same as the bore diameter specified in the SAMMI Cartridge and Chamber Drawing. For the 30-06 Springfield this value is 0.300 inches. This is similar to the ‘ogive’, which refers to the forward most part of the bullet having the base diameter; this is 0.309 inches for the 30-06 Springfield. The bullet datum will always be slightly forward of the ogive. Both are near the middle of the bullet and neither are visible features. No one else uses this part of the bullet as a reference point so this is a unique concept. I use it because the exact position of the ogive is difficult to locate, so an alternative is needed. Almost half of a typical bullet has the ogive diameter so it's hard to know exactly where on the bullet a measurement taken from the ogive diameter is being taken from. However, there is only one place on the bullet which has the bore diameter so it's very easy to precisely define and that place is fully on the taper of the bullet so it's pragmatically easier to locate precisely.
This disclosure does not discuss how to measure a chamber in order to know what the proper distance is to set up the internal assembly of this die, nor how to measure a cartridge that is produced by this die to ensure that it is properly sized. The technology for making these measurements precisely does not exists. The challenge in both situations is that the reference point is in the middle of a conical section so there isn't a physical feature to anchor one end of a measurement device on nor any visible indicator of where it is. It is still probably possible to make this die work well through trial and error but it would be better to solve the problem on how to take a precise measurement from the center of a cone. I have developed an elegant solution to the problem but because the result is a tool of general applicability and not just a tool for measuring rifle chambers and setting up this seating die it is the subject of another disclosure.