@
tac a1 with a I:8 twist you have the advantage of being able to use heavy for caliber bullets - up to 80 grn ELDM and Targex etc. I think that a 16" barrel handicaps that advantage. I would go for a 18" and personally I favour 20". Have a look at @
gimp's Howa on the .223 hunting thread - its 18" and is a neat little package and I think he tops 2800 fps with 80grn.
You can still shoot light bullets with a 1:8 if you want to.
I notice that on here the people with really short barrels use lighter bullets to keep the speed up which with a 1:8 barrel to me seems to be counter intuitive.