Looks like they're using the cannon to make both puffed rice (the first "shot") and popcorn (the second one). Same principle, of course.

They were working with rice, after firing that batch they load the cannon with flint corn (pop corn)

Corn explosion is louder than rice one.

I'm normally good at picking up on sarcasm on the internet, this is just an advanced joke, right?

Wow, that is literally a cannon ... crazy.