Because it's a bong, and it's in the shape of a shotgun, much like the one Kurt Cobain ended his life with. I assume the barrel of the shotgun is where you put your mouth, to fit with the theme.

If your bong goes "bang", you're toking on an actual firearm. See: salient reference.

I'm not sure how high you are right now, but it's called the Kurt CoBong because it's a bong.

Upsetting Nirvana fans since 1994