What, pray tell, do you think is in gunpowder? It's not magic dust that goes bang. It's a pre-mixed, pre-compressed mixture of fuel and oxidiser. When used in a gun, whether it's a muzzle-loader or a cartridge, the confined combustion of the mixture generates thermal energy and expanding gases that impart kinetic energy on a moving component (the piston or a projectile). The only difference is that the combustion event in an ICE is subsonic (deflagration) while the combustion in a gun is supersonic (detonation).