Elnathan - 4150 rifle barrel quality is a great choice for machine guns.
Muzzle loaders do not, in my opinion, need such a hard, wear resistant steel.
Muzzle loaders, again in my opinion, do need a DUCTILE steel. They are apt to be fired with considerable air space between powder & ball. Yes, we all know to mark our ramrods but then, who has never been distracted while loading with his short starter? Or maybe left who knows what air space?
I do not know why this happens but I do know that an air space between powder and ball has been known to cause problems for some time now. That English artillery expert, forget his name, wrote a nice article on Brown Bess muskets blowing up for this reason, about 1760 in Proceedings of the Royal Society. I read it about 1965 in Philadelphia, but have no access to Proc Roy Soc locally.
Anyway, Banish Thee the term "tensile strength" from thy vocabulary.
What matters in your muzzle loader is enough ductility that the thing will bulge, rather than shatter, when something Bad happens.
As we are all subject to human error, and explosives do very surprizing things on occasion, Bad Things are a'gonna happen.
Just a side note. In the chemical industry occasionally one wishes to make a large strong vessel, for which some grade of carbon steel would suit. Except steel would rust away in the concoction they are producing. So, make the whole thing out of Hastelloy C-276 (Cr Ni Mo &c) which co$t$.
Solution - clad a plate of Hastelloy onto your steel. Steel makes the vessel strong, Hastelloy will survive the nasty chemicals inside.
Decades ago Lukens Steel did this by hot rolling the two plates together, so they stuck.
Now they use "explosive cladding". One lays one plate down on the very flat ground, lays the second plate on top. A little wooden fence is built around the edges so one may put a nice layer of ammonium nitrate and fuel oil on top. With a cap, maybe several caps - At One End. Fire the thing, BOOM Now you have clad plate, might need to flatten it out a bit.
Except every once in a long while, for reasons no one has chosen to explain to me, the explosive wave sorta gets ahead of itself, travels along the plate and bounces off that weak little wooden fence. When that wave comes back to meet the main one, well, BOOOOM Now you have a bunch of pieces to pick up. Bye, bye plate.
I have yet to meet or read about the human being who knows why this happens. There may well be several different reasons, black powder acts one way, smokeless another, ammonium nitrate & fuel oil a third, or maybe they all have several ways of causing trouble.
All I do know is that explosives occasionally behave in ways we'd prefer to think they do not.
If it happens in that tube you are holding, better hope the steel is ductile enough to stretch a bit rather than crack into pieces. Along with the odd body part.
No, tensile strength is not the most important thing. Strictly speaking something called "Fracture Toughness" is what matters. That is a bit esoteric for me, let us just say you need ductility as measured in a tensile test. More importantly, good resistance to snapping when struck suddenly in the presence of a notch.
I will not knowingly use any free machining steel barrel. Yes, I have in the past. This whole discussion has inspired me to re-educate myself a bit.
Ya wonder why you've not heard of a few thousand modern barrels failing? Read Hatcher's Notebook Yes, actually read it.