Eterry:
You didn't say what size the barrel is at the breech or the caliber, or the size of the existing vent hole.
Without this information, I am just guessing but if the gun is a .50 caliber and the barrel measures 1" across the flats at the breech, the vent hole will be about .250 long.
If the vent hole is 1/16" in diameter that makes the hole's length 4 times longer than the hole. That's a LONG way for the flash from the pan to have to travel to get to the main powder charge. Even if the vent hole is a 5/64" diameter, a 1/4" wall makes the length of the hole 3.2 times longer than the vent diameter.
This is (IMO) the main reason a lot of the original flintlocks had a larger diameter hole cut from the inside of the barrel in line with the vent hole. Doing this shortens the length of the vent hole.
It is also the main reason vent liners have become so popular. All of the commercial vent liners sold on todays market have a larger diameter machined on the inside end of the liner to shorten the length of the actual vent.
Personally, I like the idea of installing a modern vent liner in your barrel with it located where it should be.
The most important thing to keep in mind is, the size of the tap drill for the new liners threads will have to be large enough to allow it to completely cut away or remove the existing vent hole. Only by doing this, will you end up with the complete, full threads the liner needs to seal the joint.
This probably will make a large vent liner that uses a 5/16-24 thread necessary.
Before ordering one of these, do a little layout work to see if it will work.
This can be done by first, making a mark on the side of the barrel that is in the right place for the vent. (Centered in the pan and in line with the top surface of the pan.) Then, as accuretly as you can, measure the distance from the mark to the "far side" of the existing vent hole. (The side that is farthest away from the mark.)
If this distance is less than .136" your in business. ( The tap drill for a 5/16-24 thread is a letter I, (.272 diameter) ) If it is up to 1/16" further away, you can still get the tap drill to remove all of the existing vent by moving the proposed liner location toward the existing vent hole. If that puts the new vent hole 1/16" away from the "ideal" location, don't worry about it not working. It will be close enough.
One thing to keep in mind though. If you need to locate the vent liner too far away from the "best" location there is a chance that the large 5/16-24 threads of the liner will break out of the barrel flat. From a safety standpoint, this is not a problem but, with the liner threads sticking out into the 45° flat above the side flat, it won't look very nice.
Let us know what you find and what you think you want to do.