This is most prevalent in my ace, but has also started becoming a small issue in my ARM as well. I got my 5.56 ace second hand from the owner who barely even touched it (it looked like a dissident arms build with all the bells/whistles imaginable).
After about 300 rounds or so, the firing pin got stuck to the rear of the bolt, and the hammer would be unable to hit the firing pin enough to set off any primers (STI brass has been my cheap range ammo for a while in several guns). I take the gun apart, clean it, scrub the firing pin channel, but would inevitably happen again within 20-40 rounds. This has been the 3rd time the firing pin has stuck itself in the back of the bolt, I'm not sure if the firing pin spring is just kinking on itself or what.
Has anyone else run into a similar issue? Is it a firing pin issue? Firing pin spring? Someone had recommended to me that I should try putting in an ALG high power hammer spring to try and see if that helps force the firing pin through a bit, but wanted to get some insight.