My understanding is the Kyle has his own but the one he used that night belonged to his friend.
Wisconsin rule is that under 18 can’t possess generally, but there is an exception for over 16 in that they can possess long guns.
So it was down to the length of the barrel which, in this case, was long enough for it to be characterized as a long gun.
Hence, judge dismissed the charge.
“So it was down to the length of the barrel”
Which was totally ridiculous because you are not going to walk in to a licensed gun store and walk out with a gun manufactured with a short barrel. Yes, it could have been modified but nobody ever alleged that.