potato gun

about a month ago, me and a friend decided we wanted to blow some stuff up, so, as any normal person would do, we built a potato gun. after about an hours work, we finished and decided to test it out. there is actually an enormous amount of physics in firing that thing. first of all, when the explosion occurs, the gas expands which exerts a force on the potato causing it to fly out of the potato gun. this can relate to newtons 3rd law because the potato exerts the same amount of force on the gas as the gas exerts on the potato. of course, i couldve figured out how far it shoots using the distance formula, but i dont know the initial velocity.