Something else that could be the problem is the barrel crown, or damage to it. I got a gun some years ago in a trade, and took it to the range to see if it was a shooter. It shot about a foot to the right, so I hammered the front sight over, and got it on the target, but the sight was hanging on the edge of the barrel flat. After I got it home, and started the cleaning process I notice the muzzle had a ding in it that created a small bulge in the crown. I took it out to the shop, and not wanting to waste a lot of time on a gun I was going to trade off anyway, chucked up a larger than bore round stone in my drill, and gave the crown a trueing up. A chunk of fine Emory paper and my thumb smoothed out the sharp corners, and she was ready for another trip to the range. Mission accomplished, after a couple of shots just to be sure, I moved the sight back to the center of the barrel and eventually sent her to a new home.
Hungry Horse