I don't remember the posting but here's my basic lock tuning procedure:
If it's a newly purchased lock and you're having troubles, you should return the lock and get another. But many times, we get a lock by trading, or it was purchased long ago and sat in a box of parts for a couple of years, or maybe you'll need to work on an old lock. Almost any lock can be made to work more smoothly, reliably, and with greater speed.
Won't spark well
Normally you want to get a new frizzen if you can, if she's not sparking. Likely any frizzen part you buy is going to come "as cast" and will require de-gating, polishing, drilling for the pivot screw, hardening and tempering. If a fine new file skids on your current one, it is a hard-enough frizzen. If it digs in, hardness may not be optimal and the face will gouge. A book could be written about re-hardening and tempering frizzens, "steeling" them by attaching a plate of high carbon steel, case-hardening the old way or with Kasenit, etc. You should still order a new frizzen and mess with that one so you have at least one that works, whenever possible.
General speeding up of a lock
Clearance issues
First look for anything scraping and causing friction. It's not unusual to find the mainspring tip or the frizzen spring tip rubs against the lockplate. Carefully file these back then polish the inside of the springs. The arms of these springs should "float" along the plate without ever touching.
Polishing
All surfaces that work against each other should be polished. Every axle and every "bearing"- the axles are the sear pivot screw, frizzen pivot screw, and the tumbler axle. These can all be polished but care must be taken to not reduce their radius and introduce "slop". First check for hardness- it would not hurt to case-harden the sear pivot screw and the frizzen pivot screw and draw them back to about 450 degrees in the oven for an hour before polishing. The tumbler axles will be hardened already. When polishing, back 600 grit wet or dry paper with a file and keep everything square and sharp. Note that many sears and tumblers have little raised bearings or rings of steel a little taller than the body to rotate against the lockplate and also the inside of the bridle. Be very careful to not obliterate these. Polishing these surfaces is "iffy" sometimes. If you polish the inside surface of the bridle that holds the small tumbler axle, be careful to not take much metal off or the bridle will stand clear of the tumbler, and then the tumbler legs would need shortening. Be careful to not "dish" every hole when you polish. Backing your abrasive paper or cloth and polishing by hand is the safest.
Fit of frizzen to pan
Look at the fit of the frizzen pan cover to the lock pan. Because the pivot is already set, there's not too much you can do here but sometimes, removing a little metal from the top of the pan surface near the fence gets things sealed up a little better.
Lockplates used to be case-hardened
It would be nice if the lockplate was case-hardened but many locks work for tens of thousands of shots without this feature. There are risks of bending or warping the plate when quenching, so this is probably beyond what should be done to a working lock. if you're building one from parts, consider doing this and getting a really nice bearing surface for the tumbler to rotate on.
Problems with frizzen not flipping open
By far the most common problem with flintlocks is the interplay between the cock, flint, frizzen face, and frizzen spring. You want the cock to strike the frizzen smartly, 2/3 or more of the way up, and you want that frizzen to fly open when the cock is descending, and stay open. Problems here are tricky to diagnose and fix. These coordinated actions require a matching of strength between the mainspring and frizzen spring as well as good geometry.
Things to check:
Note how far the frizzen must be thrown back before it cams and flips itself open, and how much force is required. Is there grittiness or friction there? Carefully polish the top of the frizzen spring arm where the "teat" on the frizzen rides along it. Do the same for the little "teat" and be careful to not change the angle of that cam or round everything over until there is very little contact between the frizzen "teat" and the spring.
Heavy or unsteady trigger pulls
Sear to tumbler notch engagement is critical to good and safe shooting and unless you are both skilled and cautious, this should be left alone. If you want to go there, remove the mainspring and sear spring and now look closely at how the nose of the sear interacts with the half-cock and fill-cock notches. The sear nose should be square to the length of the sear, perpendicular to the lockplate. And ithe sear nose should be thick enough to be strong, just thin enough to fit into the half-cock notch reliably. If it is too thin, worn, not square, then stoning it back can alter the geometry. If you do stone the nose, hold the sear in a vise, use a fine stone, and get it perfectly square. The same is true for the full cock notch of the tumbler. It must be perpendicular to the axle of the tumbler and have a sharp and clean indent that engages the sear nose well. A very little judicious stoning- in a vise, level, with good light and a fine stone- will fix most rough or heavy trigger pulls. Rarely, the angles of the sear nose and full cock tumbler notch do not match, and a heavy or unsafe trigger pull is the result. The trigger forcing the sear arm up, and the sear nose down, should not move the cock an iota forward or rearward. If it is "lifting" the cock, you are fighting the mainspring. This is a good idea for a musket to be fired in volley, where an 8 pound trigger pull makes it unlikely to fire by accident, even when facing artillery fire. But such a pull is not for a squirrel rifle. The other extreme- the nose slipping out of the full cock notch- is a safety nightmare. A skilled metalworker can correct bad angles there, and anyone who does not know what he is doing, can make a safe gun unsafe.
Look to the top surface of the sear and the working arm of the sear spring- these work together and both surfaces should be polished. The sear spring must not push against the "hump" that the sear screw passes through, but that problem is rarely encountered.
Those are the basics- if you need to consider bending a cock to change the angle, bending a frizzen face, installing a new stop on the cock where it strikes the top of the lockplate, making a stronger or snappier spring, or adjusting the angle of the cam on the frizzen spring, you're getting into deep woods.
