I'm going to out on a limb here and say....I believe your projectile either started out as a 45 ACP pistol round and through oxidation, corrosion, and wear is giving you this measurement. Because of the engraving of the barrel lands....you'll need to rotate the projo and take an average.
Second guess is that it started out as a smaller caliber projectile and through hitting soft dirt or water....it has upset to the slightly larger diameter that you are measuring.
If your curiosity is strong enough....stop by any friendly jewelery store, or Gun Shop and get an actual weight, if not in "grains" we'll do the math. If its #225-230 grains that would support the theory that it is a 45 ACP. With it being as rusty as it was, either steel core or jacketed and the size your measuring.....I'm going to stay with it being a 45 ACP projectile.
Tirediron