I had always thought the 357 Sig was a perfect .355 bullet in diameter, but I've read of an optimum .3555 or .356 diameter. The 1 gr. difference in bullet weight between the 9 mm and 357 Sig had me always wondering about its reason to be. Why produce different bullets of the same diameter ? One Speer expert told me it was in the design to help it perform ideally at higher velocities. Could it also be that there exists a minimally higher diameter ? I know Speer shaves their bullets once the cartridge is assembled but this is not exclusive to the 357 Sig, I think it has more to do with giving the bullet its final correc shape because of the bonding process.