As soon as the cannonball passes through the ring, its mass will go up hugely, meaning the force provided to a small pellet will barely move it any further.
Worse, the player might have just invented a squib suicide gun.
The moment the cannonball returns to normal size, it is blocking the end of the barrel, but the force of the expanding gasses from the gunpowder burning haven’t escaped the barrel yet. The barrel, for that moment, is effectively blocked and all the force of the gun will go apply equally in all directions on the barrel to explode with full force in the hand of the player.
Worse, the player might have just invented a squib suicide gun.
The moment the cannonball returns to normal size, it is blocking the end of the barrel, but the force of the expanding gasses from the gunpowder burning haven’t escaped the barrel yet. The barrel, for that moment, is effectively blocked and all the force of the gun will go apply equally in all directions on the barrel to explode with full force in the hand of the player.
It could be under the barrel and a few inches forward, like on the end of a bayonet.
It would have to be well calibrated, but it’s possible depending on the rules of the ring.
You’d still have other issues around mass and size, but gun configuration could be solved.