USA cannons that come with the 3286 are simply missing a spring. Find some vintage cannons (non usa) that should do the trick. Or Just unclick the cannon barrel in half, and find a suitable spring (certain ballpoint pen springs may fit). The spring will keep the black stem in the forward position. When you pull it back, then let it go, and it will launch the cannonball forward. Good luck finding it afterwards, though. As they are tiny!
Or you can upgrade to the newer USA cannons that have launching projectile-shaped cannonballs....but the shot has a long shaft, like an arrow, and when loaded the cannonball can be seen protruding from the muzzle. Ugly, but more practical for shooting if you ask me.