I'm curious, 70CB, why not go with an 80% lower and skip the whole FFL process? I bought a 300 Blackout full upper from Hardened Arms ($300) and an 80% lower with a jig for under $150. No serials, no paper, and about 30' of mill time.
As soon as the weather breaks, i will go thigh a break-in and cook up a recipe for it in both sub- and hyper- sonic.