Don't bend the barrel until you have checked the muzzle to see that the crown is concentric, and that the muzzle is square to the bore. You can probably move that ball over by simply filing a bit of metal off the right side of the muzzle, so that its released on that side first, and gas blowing past the right side will move the ball to the left. It doesn't take much to move that windage, so take a few strokes with a file, then fire a couple of shots, and then take a few more, and check it until you move the ball where you want it to go.
If you do have to bend the barrel, then try to find a gunsmith to do that for you.
Does this fowler have a rear sight? If so, try drifting the sight to the left to see if that won't move the ball. If you were pulling the shot a couple of inches off center at 50 yards shooting from a rest, I would think it has to do with your finger control. put the bottom side of your finger against the trigger guard, and use the trigger guard to help stabilitze the finger as you squeeze the trigger. This will put the pad of you index finger on the bottom of the trigger, rather than the center, and you will have the most movement with the least amount of trigger pull weight to fire the gun( leverage). By indexing your finger on that trigger guard, you are much less likely to jerk the trigger, or pull the gun towards your right hand side, thus moving the ball to the right. Some shooters will put their right thumb on the back side of the trigger guard to create a pinching movement to fire their guns from a bench rest. This technique also will eliminate the tendancy to flinch, or pull the trigger in anticipation of recoil.