Chawbeef,
Before I go any further, it may or probably would be best if you could return the lock to the maker/assembler to fix the problem.
I have worked with large military percussion and flint locks more than anything else over the years and found what you mentioned enough times I had to come up with something to do about it on most locks. Many of the repro military locks have "looser" fitting parts and that means there is more "slop" in the tolerance.
I found the problem you describe most often with those who shot High School or College .22 Rimfire competition and also those who were used to the super light double set trigger pulls of civilian rifles. They were not continuing the "follow through" on their trigger pull and that was part of their problem.
However, since your lock does it outside the stock, then it also is a problem with how close the sear nose is to the half cock notch as it swings past the sear nose. So here is how I go about fixing it.
First, take the mainspring, sear spring and bridle off the lock. Take the cock off the tumbler and then put the tumbler back into correct position in the hole. Then put the sear screw in the sear (alone) against the lock plate. We don't want the bridle there because it will interfere with seeing how close the sear nose is to the half cock notch.
If you are right handed, hold the lock and tumbler in your left hand. Hold the Sear in your right hand and push up on the tail so it fully engages in the full cock notch. Now slowly and carefully push up on the tail of the sear until the sear nose JUST CLEARS or goes beyond the bottom of the full cock notch. Now this is the tricky part, you want to capture or stop the movement of the sear completely at that point and hold it there with your right hand. Now gently rotate the tumbler forward and see if the sear nose will hit the half cock notch at that point. (It probably will the way you say it is acting.) This will show you where the sear nose is impacting the half cock notch and you repeat this procedure as you work on shortening the half cock notch - to check your progress.
Now, let me say one thing about this check, it is not completely realistic BECAUSE you are supposed to keep pulling the trigger beyond this point and thus moving the sear nose further away from the half cock notch. So the sear nose should never actually be that close to the half cock notch in actual operation.
Zonie is correct that chamfering/angling the bottom of the half cock notch MAY be all you need to do to give just enough clearance for the sear nose to pass by the half cock notch. The angle is further to the forward area of the tumbler on the bottom and closer to the end of the half cock notch on the top. Hope that is clear?
However, I have run into many locks where adding that angle still did not give quite enough clearance for the sear nose to pass by the half cock notch. Then you have to cut the half cock notch shorter, but still maintain the angle as you shorten it.
If there is not much material you have to take off the half cock notch, an India stone is used to do this. I usually use a "Three Square" or Triangular Medium India Stone when not much material has to be taken off. When I have to take a good bit of metal off the half cock notch, I use a low angle triangular diamond needle file for taking most metal off, then the India stone for smoothing it up when I have taken enough metal off. Stone/file a little at a time on the half cock and keep putting the tumbler back into the lockplate and checking to see if the sear nose clears the half cock as the tumbler rotates beyond the sear nose ”“ as explained in the check above.
When during the checks, and when it seems the sear nose is CLOSE to clearing the half cock notch when you have the sear nose captured just beyond the point it clears the full cock notch, then make sure you reassemble the lock and check it before cutting more off the half cock notch. It may not need to have enough material for the sear nose to clear the half cock notch when captured in that position.
The ONLY way to check the half cock notch holds correctly is fully assemble the lock to the gun and then cock the lock to half cock, then turn the gun upside down and hold the gun up off the ground with just one finger under the trigger. As long as the cock/hammer doesn’t slip off, the half cock notch is holding correctly. Any more force applied to the cock/hammer beyond that is only going to damage the sear nose or half cock notch or both. For Heaven’s Sake DO NOT pound on the back of the cock/hammer when it is engaged in the half cock, as I have seen some idiots do or recommend to others over the years. If you do that, I guarantee you will damage the sear or half cock notch or both and often beyond repair. Original American Civil War Ordnance Instructions actually forbade that practice because it damaged the parts and was not/is not a valid test of the half cock notch.
Now due to improper geometry in where the holes are drilled in some Military Locks, I have had to cut down the half cock notch way too far for the sear nose to clear it in normal operation AND then the half cock won’t hold properly. I doubt that will be the case for you, but if it does happen, then let me know as I have more information on how to fix that.
Gus
