I've seen some ads for semi-auto version of subguns (Thompson and Uzi, specifically) with standard-length barrels, instead of the 16" barrels usually seen. These subgun lookalikes are called "SBR"; I would have posted this on the long gun forum, but figured the Class III crowd would know more about tax stamps, and such. What is a SBR, and how do the laws affect those of us preferring the look, even if we can't get the rock 'n roll of the real deal?