I would humbly disagree with the premises expounded so far. First, it is fundamentally impossible to have an OOB ignition in an AR. If you don't understand that, then you don't really understand how the AR works, with all due respect. I have yet to see a RIFLE barrel that was fired with an obstructed bore that wasn't ruptured. Most people that were not in the military don't know how to properly clean the BC, the bolt itself and the locking lugs and even those that know better often don't as it is a PITA. Most people hose it out with something and pull a boresnake through it and call it good. I went to photobucket and looked at the 4 pics there and also read the statement. I had to think about it for quite a while but when you understand the full mechanics of the AR, have a lot of experience (I'm old) and are a reloader, it makes sense. I think this is a case of "tolerance stacking" and the following explains why. In normal operation with the rifle held up in the firing position the extractor (internally, closed on a round) is at the 2 o'clock position (area in front of the dust cover). The extractor is the weakest part of the bolt lockup on a cartridge in battery (IB). The verbage said that there had been several fail to fires just before this took place and assuming (yes, I know, but you have to make some) the shooter was not a COMPLETE idiot and the FTFs were still complete rounds when removed, here's my take on the event. They had fired 494 rounds out of 500 and who knows how many the previous time the rifle was shot, and human nature being what it is (lazy) and probably not having the rifle properly cleaned was ONE of the causes of the problem. The other part is software error. First the FTFs were probably caused (IMHO and 33 years of military service) by a very dirty BCG and the FP hole had so much crud in it that it wouldn't fire a few rounds until it was knocked out and this would further support my contention of a dirty rifle. Next, not a single person questioned what caliber this AR was chambered in!!!!!? You can get them chambered in .223 instead of 5.56x45. Now typically this is not a big problem...typically. Now say you have a very dirty AR and some combination of the following, .223 chamber, military brass and/or brass that was too long (over spec). If the BCG goes to battery (and it does so robustly) you now have a bullet that is pinched ahead of the chamber because the headspace is too tight. Normally when an AR is fired the bullet goes down the bore and when it reaches the gas port the gas blows back the BC (gas impingement, GI) until it pulls the bolt out of battery and the remaining gas pressure pushes the bolt face back into the ALREADY moving BC and the entire BCG moves back into the loving arms of the buffer and spring assy. This did not happen. The picture of the bullet shows the ring on the ogive from the cleaning rod used to remove it. It has soot on it ahead of the cannelure from blowby gases (blowby happens with every bullet fired watch one in slo-mo) highlighting the rifle engraving where the bullet was pushed into the rifling. The round went off and overpressured the receiver ring and caused the case rupture at the extractor position, blowing it off. The rupture is very specific, if it had been an OOB ignition it would have ruptured in a longitudinal direction or more generally around the head of the cartridge causing some head separation. The damage to the UR in front of the dust cover confirms this. The over pressure FORCED the bolt open and slammed it into the BC causing it to "flower" since the GI system had not functioned (the bullet basically remained stationary and became a plug, no gas no GI cycle) and started the BC into backward motion thus "softening" the recoil of the bolt as intended. The now deformed bolt was sent backward under high pressure and basically destroyed the UR since it no longer fit the insides of the UR. This sequence of events would account for the broken extractor and matching damage in front of the dust cover on UR, the "flowered" (sic) BC and the corresponding fact that the GI system did not work, the fact that the barrel is not ruptured, the fact that you still have a pristine bullet, the fact of the VERY specific case rupture matching (accounting for) the broken extractor. This is the only circumstance that accounts for everything. The BC being AFU was what was hard to figure out. If it had been an obstructed bore, I don't think it would have damaged the BC so badly if at all. This was caused by a very overpressured round, to such a degree that it's not achievable with an overloaded round. The BC is made of solid billet steel!!!! In 33 years and a lot of civilian ones I have never seen a BC destroyed. Bolts yes, but not BCs. Improperly loaded ammo (LOA) or wrong ammo (.223 vs 5.56) or combination thereof, is what caused the explosion and make no doubt, there was an explosion. This is MHO of the event. The man is lucky he wasn't shooting from the standard position. Be well and enjoy.
