they're not very frequent

Maybe once every half hour or so.

But someone is setting off something akin to an artillery shell close enough to my condo (but not in the complex) that I can almost feel it as well as hear it.

Ah, Independence Day. The Native Americans' revenge [g] (most fireworks set off locally come from stands on the Indian reservations).

I'm a big enough fan of fireworks (and will be heading off to Joint Base Lewis-McChord Monday night to watch their big show) that as long as they don't actually set anything on fire (including themselves), I'm okay with it. However, I am curious as to what exactly it is that they're setting off over there that makes such a deep boom...