Grendel LMG

[JASmith]

...My understanding is that, all other things being equal, it takes higher pressure for a smaller caliber bullet to achieve the same velocity as a bigger caliber, not less.

We have to be careful about what we mean by "other things being equal." In the present instance, the sectional density is the operative item to be kept equal simply because that quantity largely determines velocity retention and penetration potential. The .277 bore has right at ten percent more area than does the .264 meaning that a 110 gr .277 bullet will have the same sectional density as a 100 grain .264 bullet.

The smaller bore and same cartridge volume means that a small diameter bullet will see higher pressures longer than would a larger diameter bullet of the same sectional density. This is because the total cartridge and barrel volume is smaller for the smaller diameter bullet.

Restating for clarity: The same total mass of gas in a smaller volume means higher pressures.