If you look at original data:
7.62x54r: 147gr, 2886fps. 185gr, 2660fps
.30-06: 150gr, 2740fps. 172gr, 2640fps
The walls of the .30-06 are strait to the bottleneck. The x54r are tappered. Is it possible that the tappered cartridge could have better velosity, sure it is. Even though it is shorter. Similar to the Short mag theory today.
You can handload the x54r to over 2900fsp w/ a 150gr bullet and modern powder. But again, so can the .30-06.
.30-06, 7.62x54r, 8x57, and .303 Brit were part of an arms race in the late 1800's, early 1900's. The U.S. cartridge was the last to come out w/ a spitzer bullet in 1906.
All four are on par w/ each other w/ the .303 bringing up the rear w/ an original militay load of 174gr bullet at 2450fps. This had more to do w/ the strength of the action then the actual potental top end of the cartridge.
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