With an UNLOADED gun, put some lipstick, or marking dye (I Use inletting black I bought years ago when I didn't know how easy it is to make the stuff) to paint the top of the new nipple. Then I lower the hammer down onto the nipple to transfer the dye to the face of the hammer. When you cock the hammer back and peak under the "skirt", you are hoping to see a complete CIRCLE of dye on the face of the hammer. If you don't see such a circle, buy a grinding bit( Long thin,) for a Dremel tool- whether you own such a tool or not. You can chuck that grinder in a ordinary hand drill if you don't have a Dremel. Grind away all the existing dye, and lower the hammer again to remark the face. Check to see if the circle is becoming more complete. Grind away the high spots, represented by where the dye is transferred to from the nipple, until the hammer is making full contact with the NEW NIPPLE. You will see a complete ring, then. You also want the ring to be the same thickness of dye around the circumference.
When you have a ring of equal thickness, the hammer is Now Striking the top of the nipple equally on all points of the circle, which means you have maximized the strike to fire the percussion cap.
When the hammer is Not hitting Squarely on the nipple, eventually, the hammer peens down the high side, so that only part of the percussion cap is actually smacked between the top of the nipple and the hammer. That is one of the leading causes of misfires with percussion locks. It also is why some guys wear out nipples so quickly. Locks with uneven mating of the hammer and the nipple will usually begin requiring 2 or more hits before the cap fires. :hmm: :hatsoff: