Things you have to watch out for.
Piston gas setups (effects gas block, hand guard rail choices, and bolt carrier group choices)
Commercial vs mil spec buffer tubes.
Proprietary barrel nuts for rails and handguards.
They are others but these are some of the more common pitfalls.
Assumption is the mother of all ****ups.
Know what you know, Know what you don't know. -Paul Pedzolt
"Jack of all trades, master of none, though oftentimes better than master of one" -Mynshul