Another possibility.
There is a hole in the ball, and the OP says that his ramrod goes through it. Hopefully the hole in the ball isn't much larger than that.
Buy a long dowel (6" or more longer than the barrel length), of the right OD so it can fit snugly through the ball. It would be best if it took a slight tap with a hammer or shoving to get it to the breech.
Stick the dowel through the barrel & ball so that the end is sitting on the breech plug. Use something made of plastic or wood to reasonably center the dowel in the barrel at the muzzle end. I would probably use a large plastic bolt anchor that fits snugly in the barrel with the center drilled out. Loosely tape it in place.
Now, flip the barrel over and stick the dowel into firm ground. Not too hard, not too soft, just enough to hold it firmly in place. You might want a helper to hold the barrel down against the dowel.
Attach a short, tight-fitting piece of aquarium hose or tubing to the nipple. Charge up an air compressor (higher pressure = better). Stick a nozzle blower into the tubing, hold everything tight (including the dowel pressing against the ground), and give it a shot of compressed air.
Hopefully the dowel seals against the ball enough that enough pressure builds to shoot the ball out the barrel, riding along the dowel, and into the ground.
I think that's what I would try at this point.