Army

Crush inducing cartridge chamber

New design allows for increased and consistent case crush

Military Technology

An Army engineer has recently invented a solution to a significant and longstanding deficiency inherent to bottleneck ammunition cartridge case designs. The patented technology is available via patent license agreement to companies that would make, use, or sell it commercially.

Enlarged view of an interface of a crush-inducing cartridge chamber with an angular recess and a bottleneck ammunition cartridge. The cartridge chamber comprises a shoulder region 60 for interfacing with the shoulder portion of a cartridge case. The shoulder region further comprises a circumferential relief recess 601 extending fully around the circumference of the cartridge chamber. The relief recess 601 creates a vertex 603 at the intersection of the chamber shoulder geometry segments.

Headspace is a critical characteristic to both the firearm and ammunition designer and is the distance from the feature in the cartridge chamber that limits forward movement of the cartridge to the bolt face which limits the rearward movement of the cartridge. In production firearms, headspace is not a singular value but rather a range of values to allow for manufacturing tolerances. In addition to allowable headspace tolerance, the cartridge case itself also has a manufacturing tolerance to include acceptable deviations in the portion of its length that interacts with the headspace controlling features of the firearm mechanism.

In order to fully lock, the bolt must deform the chambered cartridge in the headspace. This deformation is known as case crush. For manually operated firearms (a bolt action rifle), if too much case crush is imposed, the operator may not be able to lock the bolt. For machine guns, excessive case crush demands may require an amount of energy that exceeds the percentage of counter-recoil energy resulting in either the locking mechanism being unable to fully lock or, if able to fully lock, reducing the firing pin impact velocity to the point of inducing cartridge misfires.

The amount of tolerance is limited by the material properties of the cartridge case and its ability to accommodate deformation without structural failure. For production purposes, it is better to impose chamber headspace limits that allow for the maximum amount of clearance as this translates into larger tolerances for the manufacture and assembly of the firearm components. The downsides to increasing clearance, however, are in accepting a decreased level of position control of the chambered cartridge as well additional structural demands on the bolt locking features due to the impact load applied by the case head to the bolt face during firing. This leads to inconsistencies in the initial launch angle of the bullet and degraded downrange accuracy.

To optimize the amount of case crush in a bottleneck cartridge chamber Army engineer Brian Hoffman devised a new design in which a circumferential relief recess is placed within the shoulder region of the chamber. The function of the relief recess is twofold. First, it creates a vertex at the intersection of the chamber shoulder geometry segments. This resulting intersection enables high contact pressure with the shoulder of the incoming cartridge case as the bolt moves forward during chambering and locking. This high contact pressure causes a controlled deformation of the cartridge case with the resulting case deflection occurring in the shoulder area and originating about the swept vertex. This deformation is achievable without the need for additional input force from the bolt.

Second, the relief recess creates a physical space for the deforming section of the incoming cartridge case, This is equally as important as inducing the deformation, as both characteristics are necessary in order to accommodate the increased levels of case crush without an increase to the input force and energy.

The underlying intent of this new chamber feature is to enable an additional amount of case crush during bolt lock-up given the same amount of input energy as compared to what is typically achievable with traditional chamber geometry.

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