This is a common misconception.

Due to powders of the day, the .276 was limited to a performance level not much better than the 7x57 using modern powders.

See Labbett & Mead's work on the .276 on milsurps. 150 grain bullets are shown at slightly less than 2800 fps to somewhat less than 2900 fps and that with the long barrel of the test rifles.

I achieved 2800 fps with no pressure signs at all using 150 grain bullets in a 22" bbl Ruger 7x57. The .276 was no 7mm Remington Magnum of today, but was pretty impressive in its day.

That 7x57 of ours accounted for deer and elk and varmints, and was a sheer killer. There are no flies on the 150/2800 combination, that's for sure!