Straight holes less than 3/32 are less reliable if there is much barrel wall. 3/32 was what was mostly used in the 1960s it would seem. But these are so large that its impossible to keep the charge in the barrel and will self-prime if the vent is not blocked by the pan cover.
Vents with large external cones tend to flash when dirty so they need to be cleaned. They also are harder to cover with the pan cover
The flat faced, internal cone is the most reliable in the field. Its what the English settled on it would seem and they did the serious legwork in FLs and patented a lot of stuff, most of which was simply "stuff" but some, the Nock breech and the recessed breech by Manton had validity and perhaps some others did as well.
When it get cold, when it gets windy etc etc this is when the touch hole gets tested.
I tend to follow the lead of the English. Find a good Manton shotgun and look at the locks and vents. These are still state of the art. But remember that some were set up to self-prime.
The English spent a lot of time on speed, reliability and consistency since wing shooting was a big sport and poor performance would make this very difficult.
We have to remember one thing. The main charge is ignited by RADIANT HEAT. There is no flash of fire into the vent for any distance since the bore is SEALED when loaded and it would take considerable pressure to do this. The primer flash is very low pressure since the pan is open to the atmosphere. So the more heat the main charge "sees" through the vent. The closer the charge is to the pan the more heat it sees. Larger vents work the same way. But a vent like this
Is pretty hard to fault.
I tend to build my own liners similar to WL
Dan
