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Two Letters Re: An M1A Rifle Goes Ka-boom!

Dear Jim,
I don’t suspect a squib load as the problem for the M1A. A squib in a gas-operated semi-auto generally means no cycling of the bolt or ejection, which always indicates a problem.
While there’s a link to an analysis that shows a flawed barrel, and I agree with it from the images shown, I also suspect an ammunition problem.
Consider that from the image, the chamber split, and split fast. No bulge, no crack, just a boom. This indicates a substantial overpressure in the chamber.
There are likely several things that can cause this. Two that come to mind are decaying powder or a weak primer.
The ideal cycle for a cartridge is for the primer ignition to fire up the center of the round, and the propellant to ignite from the inside out in an even burn. This is the purpose of larger primers for larger rounds–ANY primer will ignite the propellant. The trick is to ignite it properly.
If the charge is decayed or settled badly, or the primer is weak, what can happen is a “Deflagration.” The propellant burns more slowly than it should, from either base to throat or from one side to the other, and compresses the remaining propellant, thus increasing the burn rate. Rather than 50,000 PSI or so, it is possible to exceed several hundred thousand PSI from the increasing wave–there’s solid metal on one side, an expanding pressure front on the other, and the propellant in between is increasingly compressed.
This is exactly how a FAE [1] works, or why a grain silo can blow apart.
The case split from the front and didn’t just separate the head, which I think is further evidence of this. Also, the green around the primer on the bad round doesn’t seem to match the lacquer on the comparison case. It could be an indication of calcium buildup from decay, which was more common with ammo from the 1950s and 1960s.
It’s not a common occurrence, but certainly something to be aware of with old ammo. Check the condition before shooting. – Michael Z. Williamson [2]

Just a little information, a few years back, while I was working for a gun shop we got in some surplus ammo from a major distributor, that unbeknownst to them had been tumbled to clean the brass. By tumbling the loaded rounds the size of the powder changed and you got an explosion instead of a burn, thus a ruined firearm, just thought you might want to pass this along, it might be the reason?
Thanks for all you do – JAH