Way back when I started shooting and building muzzleloaders, I was given, what today I would consider a treasure trove of muzzleloading rifle barrels. Virtually every one of them had been through a house fire, and was rusty, bent, or both. I soaked them good in diesel, and after a while was able to unbreech most of them. It gave me an unusually clear picture of how early guns were breeched by hand. All but the shotguns/fowlers had breech plugs much larger in diameter than we use today, and the barrels were for the most part larger as well. Almost all of them had courser threads than the national fine threads used today as also. Many of the plugs had faces with a slight dome to the plug face, so they would pull up tight against the bore just a fraction of an inch before the threads bottomed out. Out of around twenty barrels, only one showed signs of a leaky compromised breech.
I think in some cases we are building in our own degree of difficulty.
Hungry Horse