Coilumbo really can't convict him of anything at that juncture so in essence he hung himself The firecracker idea was really quite brainless too. Anytime you blow off a firecracker it leaves shreds of paper and evidence all over the place. So that was stupidity as well.

Also, would it sound just like a gun?

It reminds me of a line in SLEUTH. The Andrew character (who's a mystery writer) says about something that sounded like a gunshot, "Couldn't it have been a car backfiring?"
The police detective says, "No, sir, If you don't mind my saying, that's only in stories."