There is no government agency that will do this for you.
You are on your own.
This is, quite literally, unbelievable for an American.
I could offer examples from my own experience in Europe and Asia but that is off the subject.
Nevertheless, please do not sue me, but I would suggest you might maybe consider using Aerospace Quality bar in either grade 8620 or grade 4130.
There are probably a number of warehouses that distribute
aerospace quality material. Pretty decent quality metal, people generally don't want their planes to fall down.
One place I ran across is Benedict-Miller
http://www.benedict-miller.com/content.cfm/AQ-Steel/4130-E4130-Alloy-Steel/category_id/102/page_id/114In my personal, though rarely humble, opinion a muzzle loader barrel has more in common with a modern shotgun barrel than it does a modern rifle barrel.
And that is, both muzzle loaders and shotguns are liable to have bore obstructions. Shotguns with snow or mud from hunting. Muzzle loaders because we all know that human beings, even those who claim to be knowledgeable, will now & again load with the ball off the powder. Maybe a little (which may double pressure), not uncommonly short started, and once with me just pressed into the muzzle (pistol, did no harm)
Ductility, ductility, ductility. The better modern shotgun barrels, I believe, are some version of 4130 or 4130 with vanadium for finer grain. My GUESS is such an alloy would not be quite up to handing the wear of modern rifle bullets.
4150 is great for machine guns where the rifling should survive thousands of high velocity copper jacketed bullets before wearing out. I'd sure it would be just fine for a muzzle loader but in my own opinion - all metallurgists have different opinions - it offers no technical advantage.
Ahh, but if the barrel maker has access to rifle barrel quality 4150 then it would make sense for him to go with it.
AVAILABILTY is also a material property. Yeah, that is my distributor (warehouse) experience speaking.
There is no organization that will do this for the muzzle loaders.
Are you guys really all so very young that you'd think it beneficial for some gov't agency to help?