Found a 5.56 barrel that is 20" long- but has a carbine length gas system on it. It is a barrel that came from the manufacturer- and not aftermarket-- (the barrel is stamped "Mossberg", so not a company that you would expect to get it right when it comes to an AR.) But before I filed it away as one of the dumber things that I've seen...... Is there any purpose that it would make sense?