I bought a Caywood Mountain Rifle, used, specifically because that company claims to use Ed Rayle 8620 barrels.
8620, even annealed, for rifle barrels is infinitely superior to 1137. 8620 is tough and ductile, so if something goes wrong it will bulge or tear, rather than shatter.
For muzzle loading rifle barrels, and for shotgun barrels, "hard" is not what is desired, "tough" or "ductile" are the properties one wants.
My working life was spent as a metallurgist, involved with everything from plain carbon steel through nickel base superalloys.
By the way, things do go wrong when shooting muzzle loaders. Humans err, even when they know better. The most dangerous error (barring use of smokeless, which is more like insanity than error) is to leave an air space between powder and ball. Sometimes nothing happens. Sometimes the barrel rings or bulges (that is good) & one needs a new barrel. And, on unfortunate occasion, the shooter will require a new hand.
8620 is the best steel in use today for muzzle loading barrels.