Just an additional observation to Zonie's good advice: Don't forget that there are many ML rifles and shotguns that were made in the 19th Century, and even earlier that are still being used. The metal used in those barrels, and breechplugs, and vent liners, or nipples, is not the same quality that was used in the 20th century or in this 21st century. And, Old age also affects these old barrel's ability to withstand chamber pressures.
The one barrel I saw reported on in Buckskin Report, many years ago, had blown out through the first Barrel Key slot , and chattered the forearm of the stock. The close up picture of the dovetail slot showed that it was cut way too deep and that there was barely any metal at all between the outside edge of that slot, and the bore inside. The other keyways were cut at a more proper depth.
You have to be concerned not only with the outside size of the barrel, but what kinds of holes, and slots are drilled or cut in to that barrel, and how deep they go. The closer any hole or cut is to the back of the barrel where chamber pressures are going to be the highest, the more metal you want between the bottom of the slot, or drillhole.
I have seen rear sight dovetails cut way too deep into thin barrels, by over enthusiastic gun builders. The gun barrels have not ruptured, yet, but I won't be surprised if I hear that such a barrel does give way when some idiot decided to see just how much powder that gun can shoot, with a couple of conicals thrown in.
