9 inch barrels is exactly what's needed to satisfy the evil desires of those of us who shoot 9x25 Dillon.
I'm serious. It's a cartridge that's intended to be full of rifle powder anyway, and would take advantage of this really really well. Somebody, maybe not the guys who did the OP's barrel, but almost anyone set up to cut rifling, should coordinate a project to make a short run of such barrels and there'd probably be several of us on GT who'd pitch in money to get one of them. Or crowd source it or something. Me, and I'll bet other guys too, would be willing to create any needed CAD models & provide a really uber-cool looking spiral fluted design for free, plus a threaded version. I could even get all the other machining done locally with people I know & who's work I trust, but I don't have any personal connections to a rifling source and wouldn't want to be the front man on something like this just to be test flying an untried vendor on something that's as make or break for the whole project, as rifling is on this one. If I'm going to do any more than throw two or three hundred bucks into the idea & let somebody else build it, I'd want to make sure these come out as match grade barrels good for shooting varmints at 75 yards and coming as close to the exploding ground hog experience as an auto pistol can get.
I'm thinking broach cut rifling would be the economical choice for a small run of barrels.