The probolem with headspace
Is that the 7.62x54R is a rimmed cartridge. The "headspace" of the rifle is a measure of the space available for the case rim to fit in, and has nothing to do with the size or shape of the rest of the chamber.
Unlike a rimless round, such as the .308 (7.62x51mm/7.62 NATO) which headspaces on the dimension from the base to a point on the case shoulder, checking the headspace on a rimmed round doesn't tell you anything about the rest of the chamber. If the chamber shoulder is too far forward, but the rim dimensions are good, it would pass a check with a headpsace gauge, and still split cases on firing.
The good news is, that if your chamber is excessive or undersize, you will have problems with virtually all the ammo you shoot, not just a round or two.
To be certain of your rifle's chamber, a chamber cast must be made, and then measured against factory specs.
This can be done at home, with a little practice and the right materials, but a good gunsmith should be able to do this, and can check other things (such as the headspace) as well.
Considering all the ammo you have shot, and only had this happen once, I still vote for a bad (overpressure) round.