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My guess based on the description would be a case head failure. See these a lot on AR's. Usually with cheap ammo, but not always. The damage described would be due to the escaping gas flowing through the action and stock.
Case head failures are usually caused by a defect in the brass of the case head, but they can be a result of a headspace/setback problem. Usually there will be other problems if this is the case, and you don't mention anything like that.
I suppose the customization might be suspect, but unless something was annealed and improperly heat treated it shouldn't be. I can't imagine any reputable gunsmith doing something like that, though. I'd concentrate on any pieces of the case you can find to investigate. I'm willing to bet you'll find a good sized piece missing where it blew out.
Careful disassembly may result in the rifle being salvaged, the damage may not be as great as it seems at first glance. Hope this helps.
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With my shield or on it...
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