FWIW ...
ime, over the last few decades with investarms barrels none were ever spot welded, and any i've worked on we're removable by me. they are machined to very tight tolerances and can be removed with semi-special tools. i have removed well over a dozen investarms breech plugs from their flintlock rifles. you can see my tools and me doing just that in several posts in this sub-forum. traditions, cva, jukar and others are near impossible to remove and traditions even cautions that attempting to remove a breech plug will destroy the barrel. i avoid those brands.
HOWEVER, there is no intrinsic need for most folks to remove any trad ml breech plugs or caplock bolsters. that is, unless you dry ball and can't blow it out with compressed air or a forced-in powder charge. been there, had those problems, and plug removal was the only course of action.
side note - when you dry ball and attempt to remove with a screw worm, the action of the screw biting into the lead will expand the lead and further tighten the ball's grip to the patch, and the rifling lands (if it's not a smoothbore). two ways to help alleviate that problem is to use a small diameter screw worm and/or pre-drill a hole into the ball using a very straight dowel or carbon fiber rod that is almost the diameter of the barrel's bore, with a 1/16" to 3/32" drill bit epoxied into one end of the dowel. use the drill bit to bore a pilot hole for the screw worm. it usually works quite nicely, particularly for guys using really tightly patched balls.
with almost all offshore trad ml barrels, they're never cleaned after proofing and that residue should be cleaned out before you set off yer first shot - both the barrel and antechamber, and the flue. if it's a caplock and the clean out screw can be removed, do so to make flue cleaning easier than trying to snake down the fire hole after the nipple is removed. you can see that residue from two different investarms barrels in two of my threads.
as to how to clean any ml barrel, everyone has their own materials and process. the hot water cleaning thing - and add in the soap thing, too - has been around for decades, but not centuries. there is no realistic proven need for either hot water or soap or any other bp residue chemical. hot water expands the steel molecules and that let's in water and krap and removes any inherent "seasoning" (like why you don't scrub a cooking skillet or pan, let alone touch it with soap!).
what's needed is just plain water, because bp residue is VERY water solvent. get the residue out, that's the main idea. in both the 18th and 19th centuries that was all that was on hand and needed for both bp ml's and bp cartridge rifles. lots of really old barrels have survived quite nicely for over a century with just water cleaning,
