RE: AR-15 ?'s
From my limited experience with AR's - there are two distinct reasons not to go with a 16 in barrel - accuracy (they are still accurate tho) and noise - the 16" barrel willbe a lot louder than say a 20" barrel - and if youre worried about overall length - youve still got a collapsible stock that youre looking at - cha shorten it up a lil for whatver reason you want