I just look at a candle flame, and note the column of air inside the flame. I recall a science teacher putting a stick match quickly into that hollow cone of air, and the head of the match did NOT light, while the stick, touched by the flames rushing around the outside of the hollow cone, did light.
I do think Jim is right that when ignition first occurs, the flame explodes in all directions, and the bottom of the frizzen is the only " restriction" as to the direction of travel of that flame. Because of the angle of the frizzen to the pan, there is some reflection of the heat and flame back towards the pan. The presence of the flint and jaws near the flash pan impede the movement of the flame back towards the action, so that there are only two ways for the flame to spread, and that is out, away from the barrel, and towards the vent in the barrel. Fire requires fuel, OXYGEN and heat. The burning flashpowder is both the fuel and the heat. The oxygen is partially provided by the chemistry of the powder itself, but the fire EATS the oxygen in the air around the powder, too. The "free air " inside the vent hole helps to draw the heat into the barrel, where it ignites the powder in the barrel.
All this happens in milliseconds- almost too fast to see. Larry Pletcher has done us all a terrific service by using High speed cameras to take pictures of the firing of these locks. He is the one who has shown conclusively that putting the powder next to the barrel, and under the vent hole produces the fastest ignition.
If you happen to have a lock where the vent hole is low, in the pan, you have no choice but to bank the flash powder away from the vent, but keep it as near as possible to speed the ignition. Its really not that difficult to learn how much powder to put in such a pan, and bank it towards the outside, so that only a small amount of powder is located near the vent hole, while the rest of the pan has increasingly more powder available to ignite. :thumbsup:
