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Don't know if this going to be of any help but years ago I had that same problem with a AR180. I took the gun to the original armalite In Costa Mesa Ca., Eugene Stoner was there and worked on the gun himself. Before he even took the gun apart he asked what ammo we had been shooting. I don't remember the brand but what he told me and turned out to be correct was that some primers were made of softer metal than others and when fired the firing pin hole would punch out a small piece which would blow back into the bolt and jam the firing pin. Sure enough he took it apart and the was a small round piece of brass the size of the firing pin hole wedged between the firing and the inside of the bolt.
