I beg to differ.
I think an external cone also effectively reduces the distance by reducing the length of the actual touch hole. I agree that it also has the advantage of more effectively funneling the gasses into the touch hole. I believe external cones are not seen more frequently because they are simply butt ugly.
To me it is as simple as the sparks from the pan flash have a better chance of finding the touch hole .
The priming charge erupts right beside the barrel - not exactly sparks getting into the touch hole. There is a real flash. The closer the eruption to the main charge, the faster the ignition is going to be. Thick barrel, small touch hole can produce almost a fuse effect.
First class guns from the flint era had an erosion resistant rare metal touch hole bushing along with the patent breech. The ignition of a British Best shotgun almost rivalled a cap gun. Sophisticated lock design, best materials, strong springs, sharp flints. On the other and, with something like a military musket, there could be a distinct pause. If I gat a chance, I'll post photos of my Wilkinson 16 bore gun's lock and breech.
That is considerably more priming powder than required and the touch hole appears to be quite a bit lower than it should be. Two to three grains of priming powder is plenty in a good working lock and there are conflicting schools of thought WRT the ideal position for the powder. As I recall, a test using high speed cameras showed there to be no measurable difference in main charge ignition time.
It is pretty much universally accepted that the best location for the touch hole is centered vertically on the top of the pan and horizontally on the center of the pan.
Well, the apparent low position of the touch hole in the first photo was an optical illusion. The position does appear to be bang on in the second photo. HOWEVER, I still maintain there is no need to load the pan up with priming powder. Two to three grains will consistently do the job quite nicely. Try it, you might be pleasantly surprised. By the way, when it comes to building and shooting flinters, you have a little over ten years on me.
It's the heated gasses - you only need a little less than 500C to ignite black powder - apparently with a well sparking lock the parks can heat the air around the pan hot enough to set off a charge without a priming powder! (or so they say)
Saw your other post - don't mix powders.
I really wish I could find this article again, but it discussed a bit of this. (and I hope I remember the terminology correctly) basically what I got from the article are that there are a couple of things at play
Deflagration of a single grain (individual grain - not weight) of powder - i.e. the time it takes for a single grain to be completely consumed. Larger grains take longer than smaller ones (duh!)
Conflagration (and I am not sure that was the correct term used) or the rate at which a single grain ignites the grains surrounding it. This had something to do with the surface area of the grain itself - larger grains had more surface area but fewer grains with lower rates of deflagration surrounding them - finer grains having a greater rate of conflagration (?)
So what I took away from that being smaller grains burn completely faster (deflagrate) and have a higher rate of conflagration they create a greater volume of gasses in a shorter period of time as compared to larger grained powders. (again... duh!) and again, when confined ~ a faster pressure curve.
i.e. the same weight of powder in different granulation should create the same amount of gas - one just does it faster than the other.
Which brings me back to "time to ignition/ignition speed/etc" or the time it takes for the gasses to get from the pan to the main charge and ignite that first grain. measured in microseconds ... I'm not convinced this is noticeable by a human.
As I stated previously, I think the coning has a lot to do with the exposed area of the main charge.... a small touch hole with no cone exposes say 5 individual grains to the initial ignition source. The rate of conflagration determines how quickly the grains surrounding it start to ignite.
If you have a cone there and expose 50 grains your rate of conflagration is the same but you are starting 50 domino chains rather than 5 - the main charge will (should?) be fully consumed in a shorter period of time. i.e. "faster"
You can also leave your vent pick in when you drop the main charge and seat the ball. this makes sure the ignition flame has a clear path deep into the main charge. Works well for me. Or you can do what my friend, Harold, does - add a bit of powdered magnesium to your 4F pan powder. Works every time but is very hard on liners.