I'd just get taps in 6-32, 8-32 and maybe 10-32 (used on some longrifle locks) and have at it. FWIW I've never seen anything smaller than that 'about' the size of a #8 in any original period piece, so doubtful you'd need a #4. Just built 2 matchlock lock assemblies using #8 fasteners. Many times, like on wheellocks and snaphaunces, the post or part was put through a piece or mating part and a simple metal tapered 'peg' them held the item in place. Or, the end of the piece was peened over.
Ensure the correct tap drill. I'd not use that formula posted above ... my machinsts training wouldn't allow me to, LOL!
I also disageee with some of what was posted above. Former machinist, GE Aircraft Engines ... and former gunsmith ... and I use your common hardware store taps all the time
on basic steels or brass. Now sure, if I was tapping a known hard receiver on an expensive modern bolt action, then I'd pull out my best tooling. However, early arms are made from your mild, basic steels.
The TIP is to use GOOD technique! When drilling, never drill straight for the final size, always sneak up on it in at least 1/16th inch increments, or 32nds for smaller holes. This does things - with THE MOST IMPORTANT is that (1) it work harden the hole and (2) makes your drill bits last longer. I also always start any hole with a countersink 'center drill' bit.
When tapping, keep it SQUARE - no wobble! Once started and you feel the teeth bite, cut no more than half a turn and back off the tap until you hear/feel the chip break. Also lubricate the tap. You don't need any fancy tapping fluid, use kerosene or at least WD40. Periodically remove the tap and clear the chips. MOST time taps break is because people work hardened the piece by drilling the tap hole TOO aggressively or that they are
way too hasty and don't break the chips or clear the chips out. So be smart, be patient, and take it slow ...
PM or message me as may be needed ...
CS Center drills shown. A variety pack of the 5 sizes can be had for $20 or so online.
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