I use leather in rifle and pistol locks.
Unlike Loyalist Dave, I only use leather in Military Musket Locks. I’ve tried lead, but it did not work as well in my two Pedersoli Brown Besses, a Charleville I once owned and from working other Pedersoli and Japanese Muskets for fellow reenactors. However, I do know a few people who had very good luck with leather for a flint pad in their military muskets for reenacting.
What I’ve always done when someone had a lock that was not sparking well and there was nothing wrong with the frizzen, was to try different sized flints in the lock with different thicknesses of leather. This included the flint being tried bevel side up and down. As a leather worker, I have lots of scraps of different thickness leather, so I could choose the best thickness of leather along with the best sized flint for each lock. So when I found the correct size flint and leather thickness for another’s lock, I informed the person and wrote that information down on a card and stapled a small piece of the leather to a card to refer back to it later. I also gave at least three or four leather wraps in the correct size to the person whose lock I worked on.
When I use different size flints with different thicknesses of leather, I am adjusting the distance from the flint to the frizzen. Only lately have I realized I never tried different thicknesses of lead for the flint wrap. Maybe that is why lead has never worked for me? Sounds like some experimentation is in order for my Brown Bess.
Gus