Answer provided indirectly by American Defense Manufacturing:
Thanks for the question. Our lowers will work with standard mil-spec forged upper receivers, our ADM Upper Receivers and some billet upper receivers. The main reason some billet uppers do not work with our lower is the paddle on our bolt catch is larger than mil-spec, and some uppers are only cut to accept a mil-spec size bolt catch. We have modified a small batch of bolt catches to fit these specific billet uppers if customers requested them.
Thanks for the inquiry.
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