Smith & Wesson have just applied for a patent on their AR-15-patterned .22 rifle, the M&P15-22.

U.S. Patent Application 20100162604: LIGHTWEIGHT, LOW COST SEMI-AUTOMATIC RIFLE

A lightweight and low cost semi-automatic rifle includes an upper receiver, a lower receiver, and a forend fabricated from injection-molded polymers. The rifle permits firing .22 LR or similar low-power ammunition from an AR-15 style frame, operates on the blowback principle, and provides a fully functional bolt catch and extractor/ejector/deflector.