Hi,
The vent hole was drilled low so the mainspring could clear the rapid taper of the barrel at the breech. Also keep in mind, our notion of locating the touch hole at the "sunrise" position relative to the top of the pan was not applied to many if not most martial guns and many civilian guns. The hole was often buried a little into the pan. I believe, that combined with a fairly large touch hole resulted in more reliable ignition rather than fast ignition because the greater radiant heat after priming burns a little was more likely to ignite the charge after the barrel was fouled from previous shots. Runar, your project is going to be really neat but that kind of barrel was not used on dueling pistols. They were used on civilian "over coat" pistols, naval pistols, and sometimes military horse pistols.
dave