I was having a discussion with a friend regarding the use of short barreled ARs for entry teams. My point was the ability to easily maneuver a shorter barrel in tight spaces for CQB applications, and the 5.56mm is more versatile than the 9mm sub guns. My buddy feels that having a shorter barrel is a disadvantage due to it's inferior ballistics, and was told by a gunsite instructor that short barreled ARs don't reliably penetrate body armor. I don't know what the ballistics are out of a 10.5" barrel, 1:7 twist, with a 55gr 5.56mm, but I don't think it would make a huge difference in CQB engagements. What do you guys think? Is there any hard data on the subject?