I got the 20" and initially had problems with it. It would extract and eject fine, but wouldn't feed. The bolt didn't come back far enough to pick up another round, so my gsmith honed the inside of the bolt carrier, and now she works fine. I thought of the shorty, but I don't trust them. You can tell a lot about a guy from his cars and his guns. I keep everything original. In this case, I used a 20" in the Army, therefore 20" made more sense to me.
I did get a flat-top upper though. I like that. I want one of those 4X32 ACOG's next. The rifle is far more accurate than I am. Actually, I guess I am now wanting to be able to utilize the accuracy of the rifle to its fullest potential, so I want a decent scope.
My favorite targets are lightbulbs at 75 yards (using a rest, or a stick), and I'd like to move up to rotten fruit at 150 yards. The BM will do that, but my eyes wont. I love them and BM has a fine product.