(B2431) writes:
The X number refers to case length. There are several 7.62 diameter
rounds out there varying length. 30 carbine = 7.62 X 33, 30-06 =
7.62 X 63.

To confuse matters more, even the metric designations aren't
always based on the actual exact dimensions, and the same round
may have several designations. Eg, 7.62x53R and 7.62x54R are the
same round, the rimmed-cartidge (that's what the "R" stands
for) full-sized rifle found.

The Japanese did this better than anyone. They had at least three 7.7 mm rifle
rounds. To be fair one was for a machine gun. The type 99 rifle used another
7.7 mm.

It's a wonder more Japanese didn't get killed or maimed by their own weapons.
The nambu sidearm was an abortion. The type 99 rifle I own was unsafe to shoot
straight from the factory; it has no noticable erosion so I slugged the barrel.
The bore is a couple thousandths too big.

Dan, U.S. Air Force, retired