What is a touch hole liner and what does it do?

A touch hole liner is a threaded bushing installed in a drilled and tapped hole in the side of the barrel, where the pan powder flash ignites the main charge. It is coned on the inside so the powder charge poured down the barrel sits closer to the outside, shortening the flash channel. That helps the rifle fire a bit faster, keeps ignition more reliable, and fouls less than a plain drilled hole.