Talking strictly subsonic ammo here.
A 16" AR w/ carbine gas will cycle subs, but you have to use a specific powder and load right up to the transsonic line. Don't know about factory subs as I've always loaded my own. My first upper was carbine gas and I got it to run, but every now and then it won't lock back on an empty mag. Eventually replaced it with an upper with pistol gas.
It's better to get a barrel w/ pistol gas.
If all you will shoot is supersonic, then gas port location is a moot point.
Just choose a bullet designed to expand at 2000ish fps if you want to hunt with it.