+1, keith. I had a semi-custom gun made by a company whose name I will not give. I decided to convert it from percussion to flint about a year after I bought it. I never was able to clean that barrel completely, and suspected the reason was that the face of the plug did not rub up against the shoulder at the back of the rifling.
Well, my gun builder removed the breech and found plenty of fouling and rust in the gap, the the amount of steel between the sides of the threaded breech plug and the outside were less than half of the thickness of the grooves to the outside of the barrel.
He was angry at what he saw, and i was furious. I had sent the gun back to the maker when I first received it as a Percussion gun, because of several deficiencies. The maker did a hack job in fixing those problems, and, of course, since I did not think to remove the breech, I didn't know about the gap, nor the fact that the breech plug was Cross Threaded! :cursing: :cursing:
I ordered a .50 caliber barrel that was 13/16" across the flats. They don't make or sell these anymore, because the industry believes that they are too thin for a margin of safety. Because I was only interested in shooting round balls in this gun, I also ordered a 1:72" ROT, but received a 1"48 ROT instead! :cursing: The gun shoots accurately enough, but its not what I wanted.
So, I agree with your thought on how Breech plugs are seated. I really don't care that there are lots of old guns out there where the breeches are not seated properly. I don't want someone someday stuffing 120 grains of FFFg powder down my barrel, trying to see just how "flat" it will shoot out yonder, and blow the back end of the gun up, because there is rust eating the sides of the barrel right at the face of the plug.
Because the stock pins and hangers were already set for my gun, my gunmaker ended up cutting a washer of cold roll steel, to fill the gap between the face of the breech plug, and the shoulder in the barrel. He could not shorten the barrel to make it fit the length of the threads on the breech plug, without moving the barrel back in the stock mortise, requiring new hangers, or pin holes in the stock.