Gentlemen: If you have to knapp, the lock is not set up correctly. But here is an easy and sure way to knapp, without the fuss you all seem to be going through. Raise the frizzen up so that the edge of the flint will strike the bottom 1/8 of the vertical plate. Hold the frizzen with a finger, or some artificial stop, cock the gun, and pull the trigger. The steep angle created by the flint falling so far to hit that bottom edge will sheer off a thin edge even across, with minimum damage to the flint.
Make sure that no moving parts are rubbing against the lockplate, as the resistence also adds drag to the cock's fall. To check this, simply remove the lock from your rifle or shotgun, and examine the backside of the lockplate in good light. Rubbing will leave marks that shine at you.File, grind, and polish the parts that are rubbing, until they don't leave a mark any more. On new locks, you may find a burr or two that will be doing this.
Now, lock set up. The springs of almost all modern locks are TOO STIFF, or strong. You only need 2.5-3 lbs of feather spring pressure to hold the frizzen closed, but most feather springs will go over 10 lbs. The main spring on almost all flintlocks will be more than 25lbs. when they only need to be about 10 lbs. And, sadly, some locks still have the angle of the cock to the frizzen set so that the flint strikes the face of the frizzen at almost a 90 degree angle. That angle should be 55-60 degrees, so that the flint actually scrapes and throws chips into the pan. As is, these locks create washboard gouges in the surface of the frizzen, where sparks have to fall into the pan by luck, rather than be thrown into the pan by the stroke of the cock. And, contributing to the wash board, most flintlock shooter wrap their flints in some kind of leather, which always creates a " shock absorber" effect, rebounding just as the edge is cutting steel, so that the steel shavings are ripped away from the frizzen, and wrap around the edge. That is why this man is only getting 5-6 shots before he starts having misfires. I have seen musket locks that had such massive main springs( well over 50 lbs. in spring tension) that they literally smashed large musket flints in one blow!, making them useless for another shot. My rifle gets 80-100 shots per flint,using black English flints, with no knapping, and my new American Fowler will give me more than 75 strikes before I will need to replace it. And no knapping. Now, I do have to check the position of the flint as the edge wears down, and move it forward in the jaws of the cock, and wedge a twig behind the lead wrap to hold the flint in place. My gunmaker( Craig Witte, Fairmount, Il.) widenned the pan of my fowler so that I get sparks igniting the prime for much longer than with my narrower pan on my rifle. I intend to grind the pan open on the rifle this spring. But I have reduced the spring tension on both springs by using slow grinders, belt sanders, and even a dremel tool, continually testing the springs to determine what the tension has been reduced to by my efforts. And, I tested the lock as I reduced the tension every 5 lbs. and then every lb as I got close to my target weight. The result is a lighter action, flints that last a long time, and absolutely positive ignition every time I pull the trigger on a properly primed charge. :grin: