M193 and M855 won't fragment unless they strike their target doing 2700 fps or better. When they fragment, they make very large, nasty wounds. When they don't fragment, they merely make a deep .22 caliber hole.
Range where velocity drops to 2700 fps (these are approximate, and can vary slightly):
M193
20" = 180-200 yards
16" = 140-150 yards
14.5" = 90-100 yards
M855
20" = 140-150 yards
16" = 90-100 yards
14.5" = 40-50 yards
Rangers in Somalia went in with either 11.5" or 14.5" "CAR-15s" and M855 ammo. While they anticipated close-quarters work, they ended up in a prolonged gun battle on the streets of Mogidishu. Several of them reported very poor performance of their weapons system. Though there are a number of contributing factors (thin bodied attackers who were likely somewhat drugged up on khat, a local plant), the combination of short-barreled carbines and M855 ammo are likely the primary consideration. Lacking enough velocity to fragment at ranges beyond 50 yards, the M855 wasn't wounding the Somalis enough to put them out of the fight. The Rangers who came in via the truck convoy, who were armed with 20" M16s and M855, did not report these problems.
This is only a single incident, but it is instructive. Military .223 FMJ ammo relies mainly on velocity to be effective, and therefore shorter barrels reduce effective range. Effective range being defined (here, by me) as being the range in which the weapon system will reliably cause wounds severe enough to take an enemy out of the fight.
This is something you may with to consider when selecting a barrel.
-Troy