Definition of "sawed off shotgun"
OK, can someone give me the straight skinny on the definition of a sawed off shot gun? I know its a combo of barrel length and overall length. I've got a 870 with a factory 18.5" barrel. Its all stock. I've seen a contraption that lets you install any form of M4 style collapsable buttstock on the 870. What I'm concerned about is creating a illegal weapon by doing this and inadvertently making it too short while collapsed. Accurate details would be appreciated.