Carbine barrels seldom have a serious pitting problem in the bore, since Carbine ammo was never corrosive. I have seen them frosty but they still shot well. The biggest problem with accuracy is muzzle errosion, usually from cleaning with GI steel cleaning rods. Once the ME is above "3" accuracy tends to drop off. Using a USGI M2 FMJ 30-06 round stuck in the muzzle, 1/4" of the projectile is about an ME of 1 or less, 3/16" ME2, 1/8" ME 3, and 1/16" ME 1 (approximately)
My un-messed with Inland ME <1
https://www.milsurps.com/images/impo...62712197-1.jpg
CMP Inland I had ME @ 1.5 - 2
https://www.milsurps.com/images/impo...ndmuzzle-1.jpg