Buy what you need from Palmetto Armory. Uppers and Lowers at every quality and price point. They have M4 Carbine style uppers with barrels made by FN(Fabrique Nationale)with chrome bores and chambers and top quality receivers, and they also have less expensive Nitride barrel assys, and stainless steel as well.
They have sales all the time and if you’re patient you can snag excellent stuff at very cheap prices.
+1 for Palmetto State
Good luck with that.
I was on their web site about two hours ago looking for an Upper and everything they had was out of stock. The only Lower they had in stock were their blems but I didn't want another Lower right now. Their price/quality ratio is very good and everyone is buying them out.