Some nasty kabooms there, Gren. There have to have been some serious injuries that went with them.

This can happen when AR's are put together by people who do not know what they are doing!

I've seen a lot of uppers for sale without a bolt. I wonder how many of these fails were the result of people swapping out uppers and pairing it with a bolt without checking the headspace.

I would say that zero of these are the result of excessive headspace. If the extractor will snap over the rim, then it is safe to fire, as long as the lugs are rotated into place. Nor it is faulty assembly. Most are due to faulty ammo, or barrel obstruction. Forgetting the cam pin. High primers, firing out of battery. The AR platform is quite safe with good ammo and proper maintenance. Having witnessed hundred of thousands of rounds of it going down range. (My first job was XO in a basic training unit).

dpcd, you're probably right. I too have seen that sort of firing. I would add to it gazillions of rounds fired full auto doing IADs with full 30 round magazines and rapid magazine changes. The barrels would smoke, the gas tubes would glow cherry red, and the fiberglass/composite handguards would get too hot to hold without a glove. Barrels got shot out but I've never seen a failure like these. I can only agree something had to be amiss.

The upper is no more than a guide-way for the bolt and a scope mount point. That is why it is made from freaking aluminum alloy. It contains no pressure. The bolt head locks into the barrel breech. Failures such as these result from over-pressure case ruptures, bore obstructions, and/or out-of-battery discharges.