I am not the worlds greatest expert in flints, but I do love them to death, and I have done alot of experimentation on mine.
Like the others said, BP is best for flinters, accept no substitutes. I purchase my powders online, from a source called the Maine Powder House. They carry, Goex, a German Brand, and a Swiss Brand. Goex is a very good, all around powder. However, there is a special, 4Fg, super hot, extra premium version of flintlock priming powder available from The Maine Powder House, That is offered by The Swiss Powder Maker, it's more expensive than Goex, comming in at $27.00/lb as opposed to Goex at $15.00/lb, but, you use so very little of it, the extra cost is nominal.
Here is the URL for the Maine Powder House;
http://mainepowderhouse.com/
In addition to this, I do a couple of "special" things to increase the speed of the powder ignition, and the speed of the charge ignition. I don't use plain 4Fg powder in my pans, I mix, my 4Fg pan powder, 50/50 by volume with flash powder, which ignites very quickly and flashes with a very large, bright flash. This 50/50 mix works very well for me, I am able to get quick, reliable ignitions even out in the wild, on very damp, even rainy days. In addition to using my special mix of pan powder, I also, put 10 Grains of the flash powder, down inside the barrel, as a charge ignitor for the main charge, and then, I reduce the 2Fg charge amount, for the main charge, by 10 Grains. Now, I don't have a really fancy electronic timer, so, I haven't actually tested my results in an exacting scientific way, but, I can tell you, that type of a load, does indeed ignite faster, how much faster, is beyond my senses, but it is noticeable by human means.
In addition to my modified flash pan powder, and my somewhat modified main charge powder load, on conicals, and mini-balls, where there is no patch being used around the ball, I do two more things to insure a good solid conical firing. I punch out my own cork wads, sized to the caliber of my rifle, and also, punch out a small hole in the center of the wad, that will allow some of the charge flame to vent into the bottom of the conical, sealing the mini ball or conical to the barrel wall by the blast exposure. one side, of the cork wad, is dipped into melted bee's wax, coating the bottom of the wad in wax. The waxed side of the wad, is placed wax down, onto the top of the powder charge. With the wad on top of the powder, I then lube the barrel, prior to loading down the mini or conical. I do this with a small dallop of barrel lube, which I mix myself, 60% melted bee's wax, mixed with 20% lanoline, and 20% Crisco vegitable lard. Sometimes I will add in a couple of tablespoons of orange honey, just to improve the smell of it a bit. When I have the dallop of lubricant pushed into the barrel muzzel, I take a small tuft of gun cotton, (Nitrated Cotton, which ignites and flashes very quickly), and ram the gun cotton, and lube, down on top of the cork wad. In this way, the lube, is not in direct contact with the powder, and will not soak into the powder, and, the small ignition hole in the base of the wad, will ignite the gun cotton, and do two things, A) vaporized the lube in the base of the mini, and B) assist in sealing the bottom edge of the mini-ball against the barrel. Naturally, the mini-ball is loaded down on top of this entire arrangement.
The flash powders, and gun cotton, is available from a pyrotechnical supplier known as Starlight.
The following URL will take you to the page that has those items on it.
http://www.starlight.com/pyro.html
In addition to the gun cotton (Nitrated Cotton) and various types of Flash Powder, Starlight also carries Flash Paper, (Nitrated Paper), which is used widely by BP enthusiasts to make their own paper cartridges, for both pistols and rifles/muskets. It is very handy, to have these paper cartridges made up with pre-measured powder already in them, as opposed to having to field measure, each load, out in the boonies. The flash paper, like the gun cotton, and the flash powder, ignites completely.
All of this may seem a bit excessive to alot of people out there, but, heck, I enjoy making a big fuss over things, trying to find the best possible way to get the most performance I can out of my flinters, and still stay well within the design limits of the barrels and max loads.
Sincerely,
ElvinWarrior... aka... David, "EW"