Too many people worry about the breech plug of the gun. Here are the simple facts:

Most breech plugs today are longer and have finer threads than most original guns I've inspected. Finer threads work fine with today's steels and give more surface to surface contact. On old wrought iron barrels a coarser. deeper thread was used due to the grain of the metal. Also the threaded length almost all of the original breech plugs were only 1/2" long. That is all that was necessary and it kept the lock as far to the rear of the barrel as possible so the pan fence of a flint lock was even with the rear of the barrel. The average breech plug today is 5/8" long or more which means the lock must be moved forward so the touch hole is forward of the breech face. This "overkill" on plug length makes the lock area hard to look proper compared with original guns. And with percussion guns that plug length "overkill" is ridiculous when you think about the minimal thread contact of a drum and nipple.  At best there is 3 to 4 very fine threads holding that drum or nipple in place, withstanding a huge pressure curve when the gun is fired. With the hardened nipples being used today, if they are not seated tightly, the threads can be broken from the force of the hammer blows creating a chance of the nipple blowing out. If the nipple is removed every time the gun is cleaned, the chance for cross-threading becomes a problem weakening the threads. At least with a touch hole liner, the liner is reinforced by the lock bolster bolted tight against it and is not removed until it is worn out.

Percussion guns can be scary. I have seen many a nipple go flying when the gun was fired. I have never seen or even heard of a touch hole liner flying out. In my opinion it's time to wean your friends off the nipple.