(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. The Russian equivilent to the 30-06 X 54R.

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.