It's fair to assume it isn't an actual explosive. That little stick of dynamite is around the size of a ketchup packet. It's definitely going to explode if Gokudera fires that cannon, but it's okay, it will just wait there patiently behind Peter.

That cannon strapped to the boy's arm has a skull facing out on both ends, and a tiny set of arms. Red flames gush out the sides of the barrel, and he points it at Peter until he starts to circle him. Then, Gokudera just hums, shrugs, then punches one of the rings on his right hand into a box on his waist. Out of it pops seven transparent discs. They grow and shrink, and some of them just act and look like rings of black bones. They levitate, rotate, spin, and one settles behind Peter, following him.

"Gotta keep the spectators safe," Gokudera says.