After doing some research I came to the conclusion it is legal to shoulder the sig arm brace. Say I bought this arm brace and was shouldering it on a 11.5'' barrel ar15. Do you guys think someone would question me at the range? Would it be smart to keep the letter from the atf to sig printed out and in the car? Whats your guys opinions on this? I am trying to what size upper to get? This seems like it would be more fun to shoot but less practical.