Check the gas port before building the gun.
A friend of mine had a local shop build a gun for him and they used an 18" Wilson barrel(looked like SPRish profile, but not sure). Guy was pretty busy and said "hey can you take this gun and break it in and zero my scope" and he gave me a few boxes of 77g FGMM ammo to use. Got to the range, wouldnt cycle. Was on a new lower as well so I swapped uppers on to a known good lower(factory DD lower) still no go. Tried my DD upper on his lower and it functioned fine. Told him something was up, but I wasnt about to tear down somebody else's work. He took it back to the shop and the gas port was WAY WAY under sized. I never asked him HOW under sized it was(he probably doesnt know anyway), but Wilson replaced the barrel. Supposedly Wilson said a batch got out with under sized gas ports. This was last summer(2013). He says it shoots great now with the new barrel.