Yes, we are drifting of-topic.
Well, I suppose there’s a first time for everything.
Jerry – To get back to your original post,
In testing a half dozen tests do not disprove 200 years of shooting in the field experience.
If vent liners truly offered an improvement, it seems to me that they would be common in American long rifles. Consider only high end rifles, where the cost of adding a vent liner would have been trivial to the cost of the entire rifle. I suppose then, as now, the people who bought high end rifles wanted only the best, and didn’t care what they cost. But vent liners in high end rifles are somewhere between exceedingly rare and nonexistent.
Even in a common rifle, say one with little or no carving or inlays, the added cost of a vent liner is still small, and I would think well within the purchasing power of the average person.
The 200 years of field experience you cite clearly shows a preference for no vent liner.
So why is that? I could only guess, but Acers comment about self priming may point us in the right direction.
Once a flash hole gets large enough, it is going to perform a whole lot like a coned flash liner – that is, give good multiple shot ignition reliability. Lets suppose the optimal flash hole diameter is near the point where self priming occurs. Under modern shooting range conditions, that’s a problem as Acer pointed out. Under the conditions that existed in the Way Back Times, it may not have been a problem.
If you made a coned vent liner out of iron, it would wear out rapidly. If you could get the same performance out of a larger hole, the economics would drive you toward a simple hole. Vents certainly enlarge with time, however, that didn’t seem to be much of an issue back then. If it was a big problem, vent hole repairs would be common. Judging by the rifles we have to study, vent hole repairs were done, but they were rare.
Back then, as now, people were interested in any technological change that improved performance. Consider that it took what – maybe 5-10 years for percussion caps to nearly completely replace flintlocks. Same with cartridge rifles. Same with smokeless powder.
So I think the answer to your original question - why is it that American long rifles never had touch hole liners? - is that they didn’t need them – their need for reliable ignition was better served by simply drilling larger flash holes than we like to use.