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what ammo are you shooting and whats your rifle setup? gas system, buffer weight, all that good stuff.
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A) Since you built the rifle; do you know how/did you properly lube the rifle for your range trip?
B) Who made your barrel?
95% chance that the rifle is under-lubed, or you're shooting weak ammo with an H or H2 buffer and the ammo doesn't have enough attitude to fully cycle the BCG. Do/did you have any failures besides not locking back on empty? (any short-strokes/failures to feed?)
Tula and Herter's (re-boxed Tula) are weak ammo and don't fully cycle rifles sometimes. If that's what you were shooting, you probably need more lube. It's common for rifles to need run somewhat wet with lube for a few hundred rounds until things wear in sufficiently and everyone gets happy.