for what OP is after I'd get a smith to do exactly what I want off a blank personally...
Suggestions on AR smiths?
Noveske does build 14.5" SS match barrels that are gassed for suppressed use as well. Might take a look at least for ideas...
I'd been looking at Noveske as a possible option. I've only had one Noveske, and it was in 300 Blackout. It was a tack driver.