Nothing proprietary on the RRA 15's, unless it's their left hand version. It's their 308 size AR that's all buggered up with proprietary stuff.
RRA barrel nuts have a reputation for being extremely difficult to remove. Pack a lunch box full of patience before you get started. There's a rumor going around that RRA uses loc-tite or some other thread adhesive on them, but I haven't noticed it. The other rumor is that the Samsonite gorilla moonlights as an RRA armorer. lol
If it were me, I would get a new upper receiver to mount the new barrel on, rather than swapping barrels on the current receiver. That .223 barrel lying around would beg me to remount it on a receiver, anyway. Might as well just do that .300 from the start and skip the swapping step.
As for a flash hider, my favorite is still the kind that requires a $200 stamp. If that doesn't interest you, then just look for your favorite style made for a .308 rifle. Easy-peezy.