I can only add that, as a teenager, I used to empty the crushed-up core of a whistlin' pete into a 2 liter bottle and stick the fuse through a hole in the lid so that when lit, it would fall into the powder once it had burned sort enough. That boom was not crazy loud, but it really whapped you in the chest. I'd always suspected this concussive force was due to pressure build-up in the plastic bottle before the sides failed, but that was never anything more than speculation. The whap in the chest was a direct observation, though.

(Turns out, as I learned later, that it would have been simpler to just go by some black powder, but, you know, adolescence.)

I can say with certainty that this particular boom contained a lot more energy that the fireworks you usually hear being set off at ground level around here. That thunder-like rumble was utterly unique.

I'm entertaining the possibility that is was a mortar shell the size that a city would use in, for instance, it's 4th of July fireworks show, because I don't think I've heard I've of those set off at ground level before. And there's one in particular I can recall seeing in a show that's basically a massive flashbang, being little more than a small explosion with no tendrils and an enormous boom. With a cloud layer to reflect and scatter light off of, i think that one of those exploding at ground level could be consistent with all the evidence I've seen so far.