Knapping the flint on your firelock is easy and necessary to get sure-fire reliable and fast ignition. We have all had times when we needed to make a good shot, but neglected to check the flint, and had a hang fire that cost us a buck or trophy or just honour and recognition. I had Daryl take a series of pictures this morning of me knapping the flint in my little .40 Peter Berry styled rifle. You can tell by the scars on both hands that I have a lot of experience with sharp stuff...and I got them one at a time too. Daryl has a dandy right across the pad of one of his thumbs when he thought ( or wished he'd thought) that he'd wipe the dust off a freshly knapped flint in his trade gun.
The first shot is of the tool I made to service this rifle. It consists of a length of 1/4" drill rod forged into a turnscrew (fits cock's top jaw screw, lock and tang screws), and a two inch length of copper bar. I turned down the end of the rod and drilled a corresponding hole through the copper, and then silver soldered the two together. No need to get fancy, though there's room for doing so here. I made one similar and polished it, then engraved it with the date for my blanket prize at our local shoot a couple weeks ago. One end is filed to a blunt chisel to concentrate the energy of the blow on a small area of the flint - the other is full. I use both as the spirit moves me.
To knapp the flint, I support the butt of the rifle at belt height under my adequate belly, and hold the rifle near the cock supported underneath with the middle, ring and little finger of my right hand - I'm left handed. The muzzle does not rest on anything, though that is a possible option with a very heavy rifle or musket. With my thumb I bring the cock just past the half cock position. This is important. If you rest the cock in the half or full bent position, sooner or later you will break the nose off your sear or the half cock notch off the tumbler. You would be delivering a blow, small as it is, to the steel on steel - recipe for damage. With the pad of my first finger I support the underside of the flint. This too is important, because you will achieve thinner and longer flakes with less of a blow this way, rather than just crushing the edge of the flint. Here you can see that the hammer falls only about 3/4".
For the purposes of this demo, I knapped the flint all the way across, though it seldom needs this. As Daryl pointed out, there is usually just a small nubby that is keeping the sharp flint from scraping the frizzen, and that's all that you need to take off. For the hunting flint, spare no expense, and do the whole edge. In the picture I have given the flint a tap on the outside corner, and removed a nice long thin flake from the underside of the flint.
$#@*, I have to run...I'll continue the description a little later today.