If proof testing a modern barrel has any value, it is to determine if there is a dangerous crack present. Any grade of steel can have a seam or crack in it; some steels are inspected for cracks before the barrel is made. Very few, if any, muzzle loading gun barrels are inspected for defects in the steel.
If you are concerned about safety I would offer for your consideration you ought to know exactly what grade of steel is used in your barrel. A large number of American muzzle loading barrel makers use a grade designed for maximum machinability. That is, it makes very crumbly chips. It is not suited for impact loading. It is called here AISI 12L14
No one likes to hear this, few believe it. Nevertheless it is true.
Green Mountain says they make their barrels of gunbarrel quality 1137MOD, tolerable for black powder.
Ed Rayl, West Virginia, is said to use a low carbon nickel-chromium-moly steel, in the USA it is called AISI 8620, which is strong, tough and ductile. "Tough" is very good when something bad happens, that "something" that is not supposed to happen.