Robert B. Davies has applied for a patent (#20110209377) for an interesting AR-15 modification. He proposes inserting a spring loaded counter weight into the back of the bolt carrier. This combined with a complicated hybrid spring/hydraulic bolt buffer should, according to the inventor, reduce muzzle rise.
The patent is to technical for me to comment if this system would work as described or not. It looks to me like it would be a lot less effective than the balanced recoil system of the AK-107 / AK-108 and the AEK-971. Also, the bolt buffer looks to me, a layman, as far too complex to maintain and too costly to manufacture. Still, it has given me a few ideas … 😉