Done properly it will make the lock work as it should. Like anything else, if done improperly it could make it dangerous, or non-functional.
Hammer gets stuck on the half cock: diagnosis?
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No I am not the half cock notch was very deep to start with. Many of the cheaper early guns had no half cock notch. As it is now I can put the gun in half cock and hang the weight of the gun off my finger and it will not move. This is a common safety check for reanactors. As a master machinist and a tool maker when I reduce the depth of something I am talking about thousands of an inch at a time
Thinking as @flconch53 is planning on taking a little bit of metal off the tumbler at the half cock notch to reduce the circumference of the tumbler thus reducing the depth of the half cock notch. With a small reduction of the tumbler at the half cock notch, a seat if it is riding on the tumbler will clear the half cock notch as the tumbler rotates through the firing cycle.
Another consideration is the pressure the sear spring is applying to the sear. You only want enough pressure to gently hold the sear against the tumbler as the hammer is brought to full cock. On release of a single trigger, the mainspring should have the energy to rotate the tumbler past the half cock notch while the finger is lifting the trigger lever that is holding the sear away from the circumference of the tumbler. If the trigger is gently pulled and the sear spring is strong, the sear spring will hold the nose of sear against the circumference of the tumble and push the nose of the sear into the half cock notch. Properly balanced springs will allow the nose of the sear to be above the circumference of the tumbler as the tumbler rotates through the firing cycle. No need for the fly to lift the nose of the sear away from the entry of the notch. One does need a single trigger pull greater than 2 pounds for best operation.
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In addition to what Grenadier1758 mentioned, something others probably have done for some time, but I was never taught; is to shorten the half cock so the face of the sear barely clears the half cock at the point the sear face just clears the bottom of the full cock notch. I've done this on between at least 300 to as many as 400 military locks that don't have half cock notches. That may be as clear as mud, so allow me to explain.
Once you have disassembled the lock down to the plate, put the tumbler (without the cock or hammer) in the bare plate and hold it in place with the bridle and screw. Then put the sear and sear screw in place. It's a little tricky but once you rotate the sear just enough it clears the full cock notch, hold the sear in that position and try to rotate the tumbler to see if the half cock notch clears the sear face as the tumbler rotates past. If it does not clear the half cock, take a bit more off the bottom of the half cock and try again.
Not knowing how much you will have to take off the bottom of the half cock notch, I urge you to take it slow and especially only maybe .005" or so at a time as you get close to clearing the sear face as the tumbler rotates past. Once you cut the half cock just enough that the sear face won't hit the half cock as you hold the sear in place and the tumbler rotates past, you are done shortening the half cock. Then I grind a bit off the now flattened half cock surface into a curved surface that ends a little bit before the bottom of the now shortened half cock notch.
What this does is ensure as long as the shooter holds the trigger the least little bit during follow through, the sear face won't hit the half cock notch again.
I came up with this as we had a bunch of new shooters each year who had come over to shooting rifle muskets from high school or college .22 rimfire shooting. They were taught to "get off the trigger" as soon as possible after the shot broke. However, that is not enough trigger follow through for a black powder lock without a fly.
Gus
