A barrel made from any type of steel can fail. We've all seen photos of modern centerfire barrels split like bananas from the muzzle to half way down the barrel and there is no gunsmith of any length of practice who has not had shotgun barrels come in that were split or blown out at the muzzle. All of these are on proof tested barrels of supposedly good barrel steels. And all of these failures are due to operator error with bad handloads, or obstructed barrels. Most of the reported cases of failure on muzzleloader barrels are due to leaving short starters in barrels or failure to seat the ball on the powder. It doesn't matter what steel the barrel is made of, operator error of this sort will result in a damaged barrel. And modern rimfire barrels are made from "mild" steels, not 4140 or 4340.
On 4140 barrels, the larger manufactures who use chrome moly steels are also set up to do proper heat treating and stress relieving of these barrels and can buy stock in large lots that has been appropriate treated to make barrels. Guys making a few barrels from 4140, i.e. most guys who make muzzleloader barrels, not only don't have the where with all to purchase large enough lots, they don't have the facilities to do the heat treating or even to do the requisite hardness testing on their stock or finished barrels.
One of Joe Manton's claims in promoting his guns was that his barrels were hydroproofed in addition to the guild required proof. This was because there could be flaws in the welds that would not cause barrel failure in the standard proof but would squirt water if you filled the barrel with water under pressure. It's not at all uncommon to see 18th century fowler barrels with patches brazed or riveted into them to fill flaws. In some cases it's obvious that the guns were used heavily after such repairs. We would never tolerate such repair but then we don't always think rationally.
Tom
