Here's how I do it...
Once the barrel and breech have been inlet, and before the lock has been inlet, I take the spring out of the lock so the hammer will flop back and forth. With a nipple in the snail, I position the lock on the side of the gun, aligning the hammer with the nipple, in the down (fired) position. I mark the front and rear extremes of the snail on the lock plate with a fine felt pen.
Then it's to the vise, and with the hammer all the way back, I file away the metal between the marks, first with a half-round course file, and then with rat tailed files etc. I return the lock to the snail with the barrel out of the stock, frequently, to check the progress. When the hammer comes down and the nipple is in the centre of the cup, I stop filing, inlet the lock plate and the lock's guts, and then reassemble the lock and check out the alignment. If you go slowly, and check frequently, you won't run into problems. Take out less than half of the metal you think with make it fit, and then check. Repeat until you're there. I've done about three hundred that way without a single oops. If anything, you'll find that once the lock is inlet, you will need to remove a little more steel from the plate so that it isn't so tight.
