... but for railguns, it's the 'cartage' that will have the rating, and the rifle will have to be approved to fire it: "Chambered for the popular XM107 round!"
.308 verses .50 BMG
But those are just numbers that would make us wonder if it will penetrate 3/4 inch thick steel plate at 200 yards (which is no and yes, respectively). However that's the only single number to look at for "ballistic performance".
I think that if or when they come to market, railguns will be (hopefully) named akin to their caliber, but they will be rated in the largest weight 'cartage' that they can chamber and reliably fire. In all actuality, it's going to be called by whatever name given by whatever manufacturer produces the most prolific round (and eventually appear as a truncated, standardized version as deemed by the government). Often, new weapons are specifically made to use a premier cartage; not the other way around - they're at the very least, made alongside (I'm so tired of video games that a new rifle drastically changes everything, all the while firing the same cartage...).
They will not be rated in muzzle velocity because as you can see, the differences between those two cartridges is negligible, but their impact is not, due to a .50 BMG being about four times the weight of a .308, and traveling at basically the same speed.
The problem with portable railguns is their imaginary power supplies. Who knows, they might be called iRifles. Whatever they're called, the bottom line I'm looking for on a projectile is energy on target.
Perhaps the nomenclature will be as thus: "Certified to fire [XXX] grain, [.XX] caliber projectiles, at the UN specified minimum speed of 3k ft/s." ... because you're relying on the rifle to accelerate the projectile, the manufacturer of the rifle will have to be acclaimed by the projectile manufacturer (whom would get to name the cartage). Call the rifle whatever you want; it has to be rated to fire (in specific calibers, obviously) a given weighted projectile (at the more or less acceptable speed of 3k ft/s).
TL;DR: some combination of weight and caliber. One tells me if it fits in my gun, the other tells me how much of a mess it's going to make.