O-ring? You don't need it, but save it just in case you do.
The biggest issue with assembling an AR isn't so much getting the parts together right - it's getting the right parts together.
US Army 1966-69, VFW Life Member, Retired Geek
Its an o-ring for under the extractor. Im running a 14.5" and mine works fine without it. I think it adds some uumph for extraction. I would run it without and see how it does, you can always add it later.
if you can manage to get it in the bolt it REALLLLLLY chucks the empties FAR
"Gun control: The theory that a woman found dead in an alley, raped and strangled with her panty hose, is somehow morally superior to a woman explaining to police how her attacker got that fatal bullet wound." L. Neil Smith
The problem with being stupid is you cannot simply decide to stop doing dumb things...
"I crapped my pants to avoid the draft!!" -Ted Nugent