The depth of the touch hole can affect ignition in some guns.
The White Lightning Vent liner, with its parabolic cone on the inside, made by Jim Chambers made a big splash several years ago when he first offered it because he made believers out of a lot of old guys who didn't think there was anything new to be offered to help flintlock ignition. We were all sceptics about his liner, until we tried it, and then found ourselves standing around at rendezvous with other old guys and bringing up the subject, only to find that others in the group had also tried the new liners, and found they were faster, and were waiting to hear what others had experienced using them. Word of mouth, with lots of smiles, and good nature laughing at ourselves became the order of the day, and Jim's business selling those vent liners soared. Not since Hot Shot nipples were offered for percussion shooters has such a small item made such a remarkable difference for MLers.
To this day, guys who have tried the White Lightning Vent hole liners laugh everytime someone asks them if they REALLY MAKE THAT MUCH DIFFERENCE! Oh, you will find people even here who claim they don't notice a real difference. But, most of us do, and have.
Now, to the question of depth/width. If you understand what a parabolic curve does, it does not take much depth or width for this surface to perform its " magic". The wider the barrel, of course, the deeper the parabolic cone will be, and the more the liner will focus both gas pressure, and heat back into the powder charge, delaying the release of gas out the vent for just a few milliseconds, but long enough to raise the pressure, and the heat to provide more complete burning of the powder, and faster ignition of ALL the powder in the barrel.
A flintlock burns linearly, like lighting a cannon fuse. It has to burn its way into the powder. By Contrast, a Percussion cap Burns a hole through the middle of the powder charge, and ignites all the powder in the barrel much faster. We are still talking milliseconds, and you need special equipment to " hear " the difference, but its there.
FFFg powder will burn more completely and faster in a percussion gun, than in a flintlock, all things being equal. FFg powder, because the granules are larger and therefore allow more air to fit between the granules in the barrel, burns more completely in a flintlock, than in a percussion rifle. If you use an OP wad in both types of guns, FFg will leave as little or less residue in the flintlock as is left in the percussion gun, all things being equal.
Because of the vent hole, velocity for the two powder charges will be higher with the percussion gun, so to equal velocities, you must use more powder in a flintlock than in the percussion- about 10% more by volume.
There are other vent liners out there, some with straight sided cones, some that are coned on the outside, and not the inside. Each has its fans. For casual shooting, it probably will not matter much which you use. For serious target shooting, the guys who shoot competitively swear that the kind of liner used DOES make a difference.
I hope this helps.
