Most flintlocks are made with a touchhole that is first sized at 1/16". That is a pretty small orifice. If there are problems with hangfires, the size of the hole is increased. Use the Numbered drill bits, as you get several sizes to use between bits measured in 64ths of an inch. I have a rifle that continued to give me problems with hangfires and misfires until I finally took it up to 5/64", at my gunmaker's recommendation.
At this larger diameter some FFFg powder can be blown out of the chamber into the pan, or on the ground, if the frizzen is open. With the larger FFFg granulation size, I have not had that problem.
There is a small drop in velocity when you go up to 5/64" from 1/16" but a few more grains of powder is all that is needed to make up the difference.
This was of concern to me because this barrel originally was made as a percussion barrel, and my fixed sights were "zeroed" for a load that used FFFg powder. When I converted the gun to flintlock, I could not get the same groups using FFFg I had gotten when it was percussion. Even raising the powder charge did not improve matters.
I began to have " hiccups"( hangfires) using FFFg, and when I changed to FFg to see if I could better groups using that powder, The hiccups continued. They stopped when I took the touch hole up to that 5/64" diameter. Then my groups came back to prior POA and size when I adjusted my powder charge up by 5 grains of FFg. At $14.00 per pound of powder, the difference in cost to me is less than 1/2 cent!
The most direct answer I can give you to your question is to Experiment with your own gun using a Chronograph to generate data. You will need to increase the size of your touch hole slowly, and do testing at each increase. If you have a liner, this may involve the sacrifice of the liner, so have another one standing by. They are not expensive to buy, however. The point at which you are going to see a change in MV will depend on the bore diameter, what projectile you shoot, what patch and lube you use, and, of course, the size of the touchhole.
The General expectation is that any flintlock will lose 10% of its MV, compared to shooting the same load combination in a percussion barrel of the same caliber, ROT, etc.
When I changed my rifle from percussion to flint, I did not yet own my chronograph, so I did not have before/after data to compare. But, I believe that the 10% rule is close enough to use.
I was shooting 60 grains of FFFg in my rifle as a percussion gun. When I switched to FFg, and flint ignition, I had to raise my powder charge by 10% to make up for the difference in burn rate of the two powders, and another 10% to allow for the gas escaping from my touch hole. That amounts to an increase of about 10-12 grains of powder, in my gun.
:hatsoff:
