CAR-15 Problems

[ODA 226]

I'm an SF Weapons man, but need a second opinion before I proceed further...

I built a CAR-15 from an NFA lower. M-4 flattop, quad rail, 11.5" barrel, pneumatic buffer.

The weapon function flawlessly in semi-auto, but when I flip it to full-auto, I get 3-4 rounds then a light primer strike resulting in a failure to fire. This only happens on full auto. Never encountered this type of problem before...

I'm thinking weak hammer spring or firing pin. Any other ideas?

226