7.62x39 reloading
I tried reloading for the cartridge in a nice all-milled Chinese SKS, with discouraging lack of accuracy. Assuming that proper American bullets and the right powder charge would reduce the spread, I could never get anything that I considered a group. Finally, noting the black ring of carbon around the bullet holes in the target paper, i decided to slug the bore and see what was going on.
A .310 diameter round ball was nice and snug tapped into the breech, and stayed snug until it cleared the rear sight base, when it loosened up considerably. When it hit the gas block, snugged up again, then loose, then tightened up a bit passing the front sight base.
My conclusion was that the factory had ensured a tight fit for these components on that very slender barrel by shrink-fitting them in place, squeezing the barrel enough to deform the inside dimensions. Given the futility of getting accuracy from such a barrel, I abandoned the attempt and the gun.
As Townsend Whelan said: "Only accurate rifles are interesting"