For the ID to decrease in size, then molecules lining the innermost aspect of the barrel would have to collapse against themselves. When heated, the exact opposite is what occurs. Those molecules expand, thus increasing the diameter of the bore.
Pretty sure I just learned something today. Now I've got to go get the 30-378wby out, 10 rounds of ammo, micrometer, and finish convincing myself, lol
N2Sooners, I could have used that bearing heater on a few farm implements, had no idea such a thing existed. Our heater was a torch!