Given how much 7.62x39 bore can vary, I can't imagine that a 0, +1, +2 type guage would be useful on the lands unless, as mentioned above, you knew where the starting point is/was. Not sure how much the muzzle would deteriorate if of the chromed variety. Military Garand held to fairly tight tolerance so things mean a bit more w/ that guage for if you're trying to determine approximately how many rounds down the pipe.
R