Looking at mine I gotta say, damn! No wonder it is slow to fire!
Mine has a .062 touch hole going not only thru the thick wall, but it doesn't break out into the bore until it's about 1/8 inch from the centerline of the bore.
How's this possible in a .50 caliber gun, you say?
The touch hole is below the breech plugs face (I use the term loosely. It doesn't have a breech plug.
Well, the bottom of the bore seems to have a large drill point going down into it. The touch hole breaks thru into this drill point and this doesn't occure until it is 1/8 of an inch inside the wall of the bore.
What all of this means is this is a prime candidate for a vent insert, but it would be best to make a home made one.
The smallest standard vent I know of is a 1/4-28 thread. This might fit, but under the centerline of the vent, they have milled away the barrel material to make room for the lock spring. This (on my gun) leaves a .141 wall of material. As the 1/4-28 vent is .125 radius, that only leaves .016 worth of material supporting the bottom of the vent liner.
If one used a #10-32 screw with a .078 hole thru its center, it could be installed and it would leave a .046 wall in this area.
Yes, it will be an interupted cut after the drill breaks into the bore and breechs coned face, but it's do-able.
I don't know about your gun, but in order to remove the lock from mine, I had to put it on half cock. This is because the mainspring tumbler link hangs down below the outside of the lock when the cock is all of the way down.
Fortunatly, Pedersoli made a little pocket in the wood for it but it does prevent removing the lock when the cock is down.
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So, to answer your question about grinding a pocket in the outside of the barrel to shorten the flash hole? I will say it won't do much if anything at all. ::
As for me, I think I'll reassemble mine and put it back up above the fireplace for right now. Another future project for my retirement.