BWesley,
You probably did see a conversion stock to make the 10/22 rifle look like the M60 machine gun. It's a plasztic or ABS casting that gives good details and mimicks the M60 in operation.
Shotgun News, I think, had ads for a molded stock that made the 10/22 look like the WWII German MG42 machine gun. The moldligs were pretty detailed, and except for the fact that the original MG42 fired from belt-fed drums or boxes, it looked pretty good.
These stocks provide visual simulation rather than actually duplicate functions. Re-enactment shooters like these things becasuse Class III (full-auto guns) is strictly regulated and outright prohibited in some places.