xkcd

Now that I think of it (far more than a silly topic actually deserves), I’m convinced it’s a 2-stroke mechanism. Loading and firing are separate stages of the normal operation cycle. Think of that like the two stages of a power cycle of a 2-stroke engine.

Piston moving up is like cannonball and gunpowder going into the barrel in the loading stage. Gasoline igniting and moving the piston down is like gunpowder burning and propelling the ball out of the barrel.