You don't have to use a S&W upper. It's hard to recommend an upper when you have not stated the exact use. What length barrel are you wanting? I always buy 20" barrels, but I shoot mine a lot from the bench or prone and out to 500 yards. I don't like the concussion of the shorter barreled AR's and I prefer the softer shooting rifle length gas systems. I'd suggest an upper of known good quality and good barrel. WOA and Larue are good performers. As are Bushmaster, Stag, RRA, Windham Weaponry, Daniel Defense, and Noveske. A lot depends on how much you want to spend too. I'd not suggest super cheap, but some here may. Are you looking for a varmint rifle upper, short barreled upper, what cartridge are you wanting? There's a lot to consider when looking for an upper. Tell us what you are looking for..
