There are many elements that make up the internal ballistics of rimfire rifles. Perhaps, one of the more overlooked aspects pertains to the firing pin. How the pin strikes the rim, its shape and even where it’s situated on the bolt face all play factors in a rimfire firearm’s accuracy.
In rimfire cartridges, the primer is contained in the rim of the case. The firing pin crushes the folded rim causing the primer to explode, which in turn ignites the powder. Smokeless powder burns generating gaseous products that expand rapidly against the base of the bullet. This force applied to the base of the bullet moves it down the barrel with increasing velocity.
As the bullet engages the rifling, it is engraved by the lands, which causes the bullet to spin. Part of the energy produced by the burning powder is used in deforming the bullet, heating the bullet and barrel, and overcoming friction between the bullet and the barrel. Although the bullet has kinetic energy because of its motion, it also has a smaller amount of energy as a result of its rotation.
There is another aspect of the impact of the firing pin on the cartridge rim that has a bearing on exactly how firing occurs. That factor is the shape of the firing pin and the corresponding shape of the dent that it makes on the cartridge base. If the firing pin hits too far toward the outside of the cartridge or hits inside the rim toward the center of the cartridge, ignition is not as efficient as when the firing pin strikes the cartridge in the optimum manner.
When you examine empty cases from cartridges that have been fired in different rimfire rifles, it is clear that there is a considerable difference in how the firing pin strikes the rim. Some firing pins have a wedge-shaped tip whereas others have rectangular or round tips. Moreover, a rather heavy but reproducible force on the firing pin is necessary to produce uniform shot-to-shot ignition.
When the rifle is held in a horizontal position, the powder rests on the bottom of the case but the firing pin on most rimfire rifles strikes the top edge of the cartridge base. Some of the highest quality target rifles have actions designed to deliver the firing pin blow to the bottom edge of the cartridge base so that the priming mixture is in better contact with the powder.
Although all of these factors must be considered by engineers when designing a firearm, particularly one designed for the highest level of competition, they are usually beyond the control of the shooter. The vast majority of rimfire shooters will never have occasion to alter these aspects of internal ballistics, but it is important to understand the basic principles.
Editor's Note: This article is taken from Gun Digest Book of .22 Rimfire, 2nd Edition.
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