Sorry for asking such a newbie questions. Or this might have been asked before. What part exactly makes the difference between full auto and semi auto AR? Is it the sear, bcg or what? Is it a drop in thingy part which can be just put in any semi auto AR? Last questions, the full auto weapons in indoor shooting ranges. Are they borrowed from NFA, Govt, ATF or the owners are ex LEO? The reason i ask this just because purely curiousity. Nothing more than that.