This is a great thread by the way. Me and a guy in my platoon were talking to a few younger soldiers just two days ago about this exact same subject. One of the guys says "cant you just file down the firing pin to make it full auto?" well, after we got through laughing we explained why that wouldnt work and then we explained how much a real, legal full auto would cost and why they cost so much.
Id spend $5000 or so to be legal anyday.