OK, found some illustrations that may help you.
When you take the lock off the rifle, does it have coil springs in it like the illustration on the bottom left of the drawing below?
http://rmcsports.com/flint.gif
If so, the following parts and numbers are germane to the discussion:
Tumbler 54
Fly 53
Sear 51
These parts are more clearly seen in the bottom right illustration of the link above, because the Bridle 49 that normally covers these parts is removed to show the parts better.
The Tumbler in the following illustration is a military lock and does not have a cut out for the fly or the fly itself. However, it still shows what I meant about the full cock notch should be perpendicular to the center of the tumbler as shown in Figure 2 and Figure 2.1 as "normal." What may be going on in your lock is shown in Figure 2.1 as "line A" where the full cock notch was either not cut right or is damaged. When the full cock notch is angled forward as in "line A," then the hammer will slip forward as you pull the trigger.
http://www.nwtskirmisher.com/graphics/sub/articles/locks-fig2.gif
Now let's go back to the bottom right illustration in the following link:
http://rmcsports.com/flint.gif
Here you can see how the Tumbler is cut out and the Fly is installed. The purpose of the Fly is to help ensure the sear nose does not bump against the half cock notch when using your set trigger. However, when a Tumbler is cut for a Fly, there is not near as much metal that is contacting the Sear Nose. So if that part of the Tumbler is not hardened/annealed correctly OR is damaged, this will also cause the hammer to slip forward as you pull the trigger.
Also and this can be VERY hard to see with the bridle and fly in place in your lock, what sometimes happens is the Sear Nose gets dinged, broken or buggered up where it contacts the solid part of the Tumbler Full Cock Notch. It can be very difficult to see this until/unless you disassemble the lock and look under the fly to see this part of the sear nose. This can also lead to the problem you are experiencing.
What I have not thought to ask was, "Have you taken the Lock out of the rifle before and tightened the Internal Lock Screws Tighter?" This will often cause problems in locks because when the lock screws are tightened too much, they wind up binding up the moving parts in the lock.
Hope this helps.
Gus
