Have the nipples been replaced with modern made ones or are they original to the gun?
One way to determine the diameter of the flash hole through the nipple is to buy you a set of oxy/acetylene torch tip cleaners. They are tiny rods (wires) with "serrations" part the way up the shank, used to run in and out of the holes in a torch tip to clean out soot or slag. The ends of them are not serrated. They are all different sizes. One of them will just barely pass through the hole in the nipple, and the next size larger won't go into it. You an extrapolate between the diameters of the two to get a very close hole size. All nipples don't have the same size flash hole anyway. They run between .020" and .030". The one thing I can tell you for certain is that the hole is nowhere near as big as a touch hole on a flinter. When the flash hole in a percussion nipple "burns out" it gets too big, and the result is too much blowback. I have seen people shooting them so burned out that the blowback would recock the hammer after ignition. It takes a cool, calm head to not flinch when that much gas is coming back out of the nipple.
SRH
