While shooting at Eagle Creek yesterday, Shootin' Buddy brought out his AR-7 which he had just picked up from his gunsmith. The rifle was an older, Charter Arms-manufactured version, which he had bought new and just had refurbished after shooting the wheels off it for many years.
After assembling the rifle he immediately began having failure-to-fire issues with the bulk .22 he was using. I can't remember off-hand, but I believe it was Federal. He switched to some of the Remington blue-label subsonic target ammunition that he had along for his suppressed MkII and one of those managed to fire.
Boy howdy, did it fire.
There was an unusually loud report and the second round didn't chamber fully. Shootin' Buddy pulled the magazine, and it had smoke floating up out of it. That's not normal.
Sure enough, the separated case head was laying on the carpet scrap at his shooting station, while the remainder of the brass was still snug in the chamber, as seen in the accompanying cell phone photo.
Monday morning quarterbacking, I'm going to guess that the sequence of "light strike, light strike, light strike, et cetera" followed by "case head separation" points to an excessive headspace situation.
In layman's terms, this would mean that the cartridge had too much room to move forward and back in the chamber when the bolt was closed. If this was indeed the case, the light strikes were caused by the round shifting forward when struck by the firing pin. The one that actually went off would have had too much room for the brass to stretch in the chamber, and it stretched past the point of failure.
Of course everybody was wearing eye protection, and nothing bad happened to the gun, but be careful when saying "Oh, it's just a .22." While its powder charge may be small, the modern high velocity .22LR chamberings operate at higher chamber pressures than .38 Special or .45ACP. SAAMI maximum allowable pressure specification for the .22 Long Rifle is 2,500 PSI more than .380, and 3,000 psi more than .32 H&R Magnum.
Remember: The eye you save may be your own.