TWR, a 16in and a 14.5 M4 have the same gas port pressure, if the guns are other wise configured the same. The longer barrel has a slightly lower pressure at the crown because it has a longer dwell time.
A 16in barrel also pushes M855's effectiveness out about another 50-75 yd. By that I mean it'll break at the cannelure like M193. Mid-length 16in is the way to go.
