I came across two interesting machine guns on the web. The first is a 10/22 Ruger built by Florida Gun Forum contributor Revolverteer into a browning-like receiver with water cooling for the barrel. It looks to be using a BMF activator to manually crank the semi-auto trigger to crank out the rounds. Looks like really nice work, and should be a blast to shoot. And it looks perfectly legal in the US.
The second gun is a dressed up 10/22 Ruger crank fire gun, by Bill Akins.