Recoil Operation - Categories - Inertia Operation

Inertia Operation

The latest design concept for recoil-operated firearms is the inertia operated system, which was first seen on the Sjögren shotgun in 1908. In a reversal of the other designs, the inertia system uses nearly the entire firearm as the recoiling component, with only the bolt remaining stationary during firing. Because of this, the inertia system is only applied to heavily recoiling firearms, particularly shotguns. Inertia operation was developed by Paolo Benelli in the early 1980s and patented in 1986. Until 2012, all inertia-operated firearms either were made by Benelli, or used a design licensed from Benelli, such as the Franchi Affinity. Then the Browning Arms Company introduced the inertia-operated A5 (trademarked as Kinematic Drive) as successor to the recently discontinued, long-recoil operated Auto-5. Both the Benelli and Browning systems are based on a rotating locking bolt, similar to that used in many gas-operated firearms.

Before firing, the bolt body is separated from the locked bolt head by a stiff spring. As the shotgun recoils after firing, inertia causes the bolt body to remain stationary while the recoiling gun and locked bolt head move rearward. This movement compresses the spring between the bolt head and bolt body, storing the energy required to cycle the action. Since the spring can only be compressed a certain amount, this limits the amount of force the spring can absorb, and provides an inherent level of self-regulation to the action, allowing a wide range of shotshells to be used, from standard to magnum loads, as long as they provide the minimum recoil level to compress the spring. Note that the shotgun must be free to recoil for this to work--the compressibility of the shooter's body is sufficient to allow this movement, but firing the shotgun from a secure position in a rest or with the stock against the ground will not allow it to recoil sufficiently to operate the mechanism. Likewise, weapons of this type must be modified (with the addition of extended magazines or stock saddled ammunition slings on shotguns, for example) with care, as any sizable increase in weapon mass can reduce the force of recoil below that required to cycle the action.

As the recoil spring returns to its uncompressed state, it pushes the bolt body backward with sufficient force to cycle the action. The bolt body unlocks and retracts the bolt head, extracts and ejects the cartridge, cocks the hammer, and compresses the return spring. Once the bolt reaches the end of its travel, the return spring provides the force to chamber the next round from the magazine, and lock the bolt closed.