No, it isn’t. You’re simply not aware of what used to be cranked out on P&W machines when they did only one caliber, only one twist all day long. There were fewer machines in operation in WWI or WWII for all the rifles we cranked out then.
The reason why these dozens of barrel manufactures seem to produce so little is that they’re doing a blizzard of bores and twists, and all that one-off work slows them down. If you set up the sine-bar P&W machines to do 1-in-10, .308 barrels all day long, you’d crank out hundreds of tubes per day (and the machines have two spindles per machine, so the operator is prepping a second while the other spindle is working on a barrel). That’s pre-WWI machines I’m talking about.
The hydraulic tracers in WWII made this speed up. You lost the flexibility of the sine bar to gain throughput by setting the machine up to do one caliber, one twist, and then did the same thing, again and again and again.
There could be many more barrel makers, btw, for an investment of $100 to $300K for the tooling apiece. Modern CNC deep hole machines make this all go faster. Why don’t we have more barrel makers? Right now, the market is pretty well equipped. I can get barrel blanks in popular calibers all day long with a phone call. By buying a dozen of the same twist/caliber at a time, all in 1.250” blanks, I get speedy turn-around.
And those are premium barrels. Want to make things really boogy? Then go to one hole/one-twist barrels on a cold forging machine - ie, give up quality for yield. This is how Remington cranks out barrels.
The technology to make barrels isn’t secret.
I remember a documentary of how the Brown Bess muskets were made, essentially wrapping flat steel around a dowel.
And a shotgun can be made damascus style with a higher grade of wire wrapped around the right size rod and either brazed or hard soldered. Or to just get a chrome-moly tube say from a roll bar.