This is a hot button topic and if you search you'll find a lot of posts. Many people have lots of ideas and we don't know that much about hard data- speed and reliability tests on the same gun, lock, barrel, with different touch holes, under the same conditions, cleaned or not, the same, each time. This is especially difficult because flints wear. If you get a slow fire, is it because the flint is throwing few sparks? Or is it the touch hole?
Most American made flinters originally had straight drilled touch holes and there are debates about the right size for best reliability, speed, and velocity. Internal coning can be done with special tools but it's not known how often this was done. Vent liners seem primarily to have been put on American flintlocks to fix an eroded touch hole.
A vent liner may or may not speed ignition, but hey, anything named, "White Lightning" will sell. Possible problems with vent liners: if not properly installed they may protrude into the bore and be a spot that grabs your jag or patch when cleaning and crud could build up. They could be difficult to clean of caked residue that could stick in the cone if you just wipe with patches and do not shoot water out of your vent. But many people who use them love them and would never build a gun without them. This makes it difficult for custom builders of longrifles to build a rifle without one. I do not believe for a minute they are unsafe unless folks have the practice of removing them, as may be done on TC's etc. Most vent liners are installed permanently and will never "come out" on their own.
I have always used them because I believed what was written about them being essential for fast and reliable ignition. I am no longer sure those "facts" are "facts".
