'Ballistically and from a physical comparison, the .30 carbine is comparable to a .357 Magnum or .38 Special pistol cartridge, '

Oh, .357 Magnum now goes 1990 FPS and has 990 Ft Lbs of energy?

'Would it be expensive to have say an unchambered AR barrel done in 30 Carbine could you use the same bolt/extractor or would it require extensive mods...! .

A .300 Blackout barrel would be great, with a chamber insert. The bolt would remain unchanged. A carbine mag can fit inside an AR mag, so that would take a little finagling, but you can see how easy it would be. At least so far; it might get more complicated with the gas port size and recoil spring weight but those can be done by trial and error. So basically, if you have a .300 upper, you can have any machinist make the chamber insert and the mag set-up should be fairly straighttforward.