16 inch vs. 14.5 inch with the FSB the same distance from the chamber:
Using the same ammunition, within normal variances, bullet will reach the gas ports at the same time.
Initial gas pulse will be the same amount of pressure on the port and the carrier.
Initial unlocking sequence will begin at the same time.
Length of gas pulse will be longer on the 16 inch barrel, as it takes the bullet longer to exit the muzzle.
The end result, with identical sized ports, will be more total energy applied over a longer time with the 16 inch barrel, which means you MAY need a larger port with the shorter barrel, or MAY get by with a smaller one on the longer one.
I'm thinking less pressure over a longer time would be a good thing. Less stress on the whole system and PROBABLY more reliable.
DISCLAIMER:
I am NOT any sort of engineer and did NOT stay at a Holiday Inn last night. This is strictly my "Armchair Commando/Engineer" opinion, and I may very well be full of it.
Lonny