Those were the Hart barrels.The way I heard it they used custom button-rifled barrels wayyy back into the Mike Walker halcyon era....
Shooting the custom Remington 40X rifle I'm re-stocking for the first time. The rifle has a blueprinted Remington 40X receiver, PTG bolt with custom engraved bolt handle, Shilen trigger, Shilen 8-groove 24" barrel, and custom stock made from Australian walnut by OP.
Trying to remember my barrel making history...didn't Mike Walker come up with the carbide button and the button rifling process and teach it to Clyde Hart?
I could swear I read that Remington made their own button rifled barrels for the 40X's, except, as noted, for using some Harts here and there.
Justin