When I cold blue, I plug the barrel with rubber stoppers, and turn the barrel on the lathe as I rub it in.
I got a [factory blued up the bore too] Douglas long chambered 260 barrel with Mauser threads.
It has been a long hard haul to break in that barrel.
It does a couple moa groups and then the flyers start.
The muzzle then looks Copper fouled.
I clean the copper out with KG12 and Flitz.
Then it is back to moa groups.