It seems our troops have the most problems with the M855 out of an M4 14" barrel. They tend to drill through and make pencil holes sometimes. The FMJ need higher velocity to upset and do damage, more like the A1 with its 20" barrel length. The closer to 3000+ fps at impact, the better. Can't get rifle performance from a long pistol barrel.
Since the troopers like compact rifles so much, I wish they'd come up with an ambidextrous bullpup design for them, much like the Kel-Tec MFB. Then, they could have a 24" barrel, lightweight and compact, and all the velocity they need.