All my liners are flat on the outside, no campher. Ignition is very fast. You want to get the main charge as close to the flash as possible. BUT, I don't think the thickness of that wall is your problem.
I still think it has to do with that patent breech and fowling.
Just me.
That is how it sounds to me as well - cleaning, SWABBING - pushes $#@* down there, makes it a wet sludge that stops the powder from reaching the flash channel. Too long a flash channel
just exacerbates the problem - especially when the inner cone is fouled as well.
The kicker here, was first shot with the new vent liner was fast - then - wiping, loading and shooting, it got slower and slower - thus, the wiping - however, I cannot for the life of me understand why someone would continue to load and shoot like that, when they do not have to. There is no good reason, aside from maybe long periods of time 10 - minutes or more, between shooting and loading the next one - as in a chunk or plank match, for all that wiping.
It really doesn't matter to us, what the humidity is - even 100% - we've shot in that, & 90% or all the way down to 6% - we do not have to wipe - ever. I have had to wipe the water out of the pan though when it is really muggy/humid, but I/we've never wipe the frizzen nor the flint - ever. I've never seen anyone in our group do this. I cannot see a need for that & no benefit to doing it.
If you want to, same as wiping between shots, every 3 or 15 or 30 for that matter, have at it. Loaded as we ALL do up here and many guys on this forum, there is no need to wipe. Our guns shoot the entire course of fire, be it 20 to 30 shots on a short day, or 60 shots on a longer one. We've even gone 100 shots in a longer day's shooting of load testing - no one wipes the bores - ever - don't have to. There is never any powder fouling buildup in the bore. Yes, it will build up in the breech, but I've never had a problem with ignition for an entire long day's shooting where I know I've fired off more than 100 rounds. That has happened with my .32, .40, .45, .58's and .69- none of them ever became 'fouled'.
We'd rather be shooting, loading and shooting, some talking, etc. Not wiping and holding up the firing line. Maybe that's how we can shoot off 30 to 50 shots in just around 3 hours. We're usually at the restaurant having lunch by 2:20 - sometimes 1:30 on shooting Sundays. We usually start at 10:00, sometimes start shooting earlier, like 9-9:30 or so. That will make for a 50 or 60 shot day. There is a reason my horns hold more than a pound of powder. My big 2F horn holds 1 1/2 pounds as I use it with my .69. Oft times I run out.
That's this horn.