I agree that the most likely explanation is the rifle fired out of battery; i.e., the cartridge was not fully chambered when it went off.
I'd be looking for a dirty chamber, possibly with a build up of fouling that prevented the cartridge from seating properly. I'd also look at the firing pin to see if it's somehow stuck forward and striking the base of the cartridge before being released or struck by the hammer.
Also inspect the trigger and sear to see if they are worn and the firing pin was released early.
Finally, if the rifle was being fired rapidly, it's possible that the shooter was too quick on the trigger OR the action is dirty and the bolt didn't close as quickly as it normally would with everything clean and lubed properly. Also check the feed mechanism; especially if it's tube fed.
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