So this happened today. And of course, I wasn't wearing a hat. It was an FN-49 in 30-06. Ammo was M2 Ball '76 HXP from the CMP. I had loaded 5 rounds in the magazine, and the last round was the one that went off. The neck is still in the rifle, as I don't have a broken case extractor. The barrel itself is (thankfully) clear. Not sure what caused this. Was it just a bad round? Bad headspace? The casings from the previous 4 rounds were all in good shape, nothing wrong at all. Any thoughts?