The flint should have it's rear portion touching the jaw screw, with the front edge (when new) almost touching the face of the frizzen. As you noted, as it wears down that distance increases. However, in most cases it's a very short change over time, and it does cause the frizzen to be struck lower on the face, but should not significantly reduce the spark production.
My flints usually last more than 30 shots, sometimes as much as 70+....including knapping, but after that such a flint might be used at a range (to be frugal), but never for hunting or in a target shooting contest, and not at a reenactment. I base those numbers off an average of 35 rounds fired each day from my musket, over a two-day reenactment. I normally install a fresh flint Saturday morning, and it's usually "done" by Sunday afternoon...., and muskets tend to be hard on flints. For my rifles, which are much better locks with much better geometry, the flints don't nearly take such a beating.
Now, flint is a natural product, so yes some will fail quickly, and sometimes you get ones that are "miracle" flints that seem to last a very long time.
IF you're chewing them up on a regular basis at less than 20 shots, meaning you cannot knap them sharp enough to continue use, I'd say perhaps you need to change some lock geometry. Try a small piece of a wooden match stick under the back end of the flint at the jaw screw to cant the front edge of the flint downward just a tad and see if that doesn't increase your flint's life.
Doing something wrong? Naw, I agree, you're simply learning how your flinter acts. Each one you use or own will be different, and have it's own quirks.
LD
Topic: Flints (Read 1058 times)