This is an interesting topic, with some great posts. The video and commentary by rfd in Reply #8 above shows excellent science. However, the person in the video mentioned that the test barrel had a White Lightnin' vent liner installed, and it looked to me as if the flash hole had been enlarged. Any comments on this? I've heard of enlarging the hole in the WL vent liner, which would result in some loss of "performance" due to the increased gas leakage, but I don't know how significant that would be in practical terms, in the field.
Jose Gordo's post (Reply #9, I think) mentioned "Pletcher's data," comparing vent liners coned on the inside versus straight holes, and showing no difference. Where can I find these data?
I had a rifle (subsequently sold) which had a 7/8" straight octagon barrel in .50 caliber. The builder installed a WL vent liner, which looked fine on the outside but protruded into the bore. This created a fouling trap and also made it very difficult to clean the breech. I was not happy with that arrangement. One individual told me the builder "should have" radiused the protruding material to match to radius of the bore... Maybe so, but whether the builder should have or shouldn't, he didn't. That rifle was a pain to clean.
I have four flintlock smoothbores with unlined, straight-drilled vents, and could tell no difference in lock time between these and the rifle with the vent liner, with the caveat that I might not be sensitive enough to detect a difference measuring in hundredths or thousandths of a second. Of the four smoothbores, one (a Caywood NW gun) was deeply coned from the outside. One (a fowler by Jackie Brown) was very lightly chamfered on the outside. The other two just have plain drilled holes.
So, my experience with this is admittedly limited, but I'm thinking the vent liner, coned from the inside, may be an advantage for a barrel with a very thick wall at the breech, but may not be as necessary and may even be a disadvantage in a thinner barrel.
Finally, in reading some of the period literature of the early 19th century, I've found a few references to "self-priming" flintlocks. Obviously, this means some powder from the main charge leaks through the touchhole during loading. This was actually considered an advantage, back in the day, especially when running buffalo. Palliser specifically stated a "self priming" flintlock was the preferred weapon for this purpose, as percussion caps could be difficult to handle while on the back of a running horse. In addition, a few people on various forums have commented on the paucity of surviving original priming horns, suggesting that the old-timers primed from the main horn. I saw a painting by Charles Deas which showed a trapper doing this very thing. However, maybe some of them didn't have to prime at all, as a separate step, if powder from the main charge leaked out the touchhole during loading. I think this may also help explain why the Canadian natives held on to their flintlock northwest guns for so long... If the guns self-primed, there would be no need to handle percussion caps with cold fingers or mittened hands.
Best regards,
Notchy Bob