most generic formulas that I have seen for .2323 based cases are around 3-35 FPS. I can't confirm this in every case, especially in an autoloader, but in my Ruger Bolt gun with 24" bbl, it seemd to hold true, as it used to be a 26"
I pick the 16 myself! For a start I dont much care for the muzzel Brakes that Bushmaster has on the 14.5 Barrels!! Down right UGLY!!! Get the 16 and have Kurt cut down the Barrel and install his Bird Cage to make it the Legal 16inch!!