I have seen a couple yt videos, the barrel length does not have a huge impact. I have a 20". A lot of ppl are killing deer with 16".
The important thing is the feed ramps. If you plan to run an AR, make sure that the feed ramps are extra wide and ideally the upper does not have M4 cuts.
I plan on getting a bolt action gun, not sure how much that matters.
take a look at the 350 Legend ammo thread. We're shooting 16" Ruger American Ranch bolt actions, 1" groups @100, killing deer out to 200 plus, haven't had a desire for anything else.