One of the really interesting variations on the AK to come out of Russian military development and testing is the balanced-recoil system, as exemplified on the military AK-107 and the commercial Kalashnikov SR-1 rifles. Contrary to common assumption, this is not a system to counteract to recoil created by the bullet leaving the barrel. Instead, it is intended to counteract the movement of the rifle caused by the bolt slamming into he rear of the receiver and into the chamber. These two events are what cause most of the movement in a 5.56mm or 5.45mm AK, not the minimal recoil of the actual bullet.
The AK-107 is the military iteration of this system, and the SR-1 is the semiauto competition rifle version, and they both use the identical counter-balanced mechanism. Basically, the gas port vents both forward and backward, pushing on two separate pistons. One moved backwards, connected to the bolt carrier, and the other moves forwards to balance the momentum of the other. The two are connected by a sort of trolley with two gear wheels which ensure the two pistons remain synchronized. This system is relatively complex to disassemble, and the Russian military opted not to adopt it.
Kalashnikov Concern then released a semiauto version called the SR-1, specifically tailored to the IPSC-style rifle competition market. It uses a plastic lower and AR-type magazines, and has a crossbolt type safety instead of the standard AK safety. It also has a very effective three-port muzzle brake. In practice, it is actually this muzzle brake which contributes the most to the flat-shooting nature of the SR-1. The counter-balanced system does work as intended, but the improvement it provides is really on par with competition-style improvements made to AR rifles – and at the cost of significantly increased weight and complexity.
The SR-1 was a commercial flop for Kalashnikov, with only one production run being made, and subject to substantial QC problems. The rifles have been discontinued, and only a small number got out into the European civilian market.