It was the same at the other end of a Bushmaster when a buddy tried to remove the commercial RE. We barely got it off because of all the gunk on the threads. Not saying that's your problem, at least I hope not, who knows what barrel heat would do to thread sealer!
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