I had no trouble grinding the face of a hammer on a Spanish made rifle, but I took it very slow, put mor marking dye on the nipple and lowered the hammer often to see how much of the high spots were removed, and what still needed work, and then used my grinding bit some more. It requires frequent testing to get it right, but its certainly a do-it-yourself- project within the capabilities of most shooters.
You can choose to leave the lock in the gun, or take it out to work on the hammer, and just put it back each time to transfer dye from the top of the nipple to the face of the hammer, after you have ground off the dye that was on the hammer the last time, You will see the ring of dye on the face grow as you remove the high spot, until you have a full ring show on the hammer. That is your proof that the hammer is now hitting the top of the nipple all around, or " Square". rather than hitting one edge of it, which beats the nipple down where the hammr strikes, and leads to misfires. My final test is to use a folded piece of paper toweling, ( 2 layers) over the nipple, and release the hammer to fall using the trigger. If a punch-like hole or ring appears in the paper towel, I am done. Because the nipple does not have a cutting edge, and the toweling has long fibers in it, don't expect to see a paper punch hole in the towel like you would if you used a real paper punch. What you should see in both layers is a good impression, 360 degrees, of the tip edge of the nipple.