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.
Yes, if you've ever seen a bayonet on a 16" carbine gas gun, you'd understand stubby. I'm pretty familiar with the 14.5 and 16" carbines gas systems but the longer barrel past the gas port actually increases pressure. That's why so many say the 14.5" with a mid length gas system is so smooth to shoot, less dwell time.
I've chronographed Colt 14.5", 16" and 20" barrels with m193 and found there's 106fps difference between the 16 and 20 and 101fps difference between the 16 and 14.5" (averaged of the 3 barrels I was testing). The 16" with a midlength system would be a better gun but it took Colt 15 years to convince the DOD to add a stiffer insert and extractor spring to the TDP, I'm afraid we'll never see a 16" M4 issued but as you pointed out, it would extend the range.
And thanks for explaining the bolt over base.
