If this upper I put together (their Shilen 6 ARC barrel and my Aero M4E1 upper receiver) shoots as good as my other uppers Mike has built me, I'll be extremely happy.
They asked me to send the upper receiver to true and make sure the barrel fits properly. I also had them include their headspaced bolt since I can't find one at the moment. In hindsight, short of the gas system and my SLR Ion Lite 13.7" handguard, I guess they are building the upper and I'm just assembling.