As 3DogMike asked for it:
Rigby indeed advertised the 3000 fps muzzle velocity of their 140 gr load in the early 1900s. At that time the “measuring” of bullet velocities was still in it’s infancy. Up to after WW2 there were no electronical devices, only mechanical ones. The gold standard was the Le Boulenge’s falling rod apparatus, but even that required a lot of guesswork and was very expensive. The readout was for an average over longer, more than 10 meter distances only and required calculations for the velocity at the muzzle. So cataloged velocities were usually guessed at and rounded up.
As Rigby had their cartridges loaded by Kynoch, a look at the Kynoch tables may help. In their 1925 catalog Kynoch listed the 7 mm (.276) Mauser Rifle 140 gr pointed bullet load with 43 gr smokeless at 2800 fps, no barrel length given.
An undated ca. 1930 ICI/Kynoch catalog gives these loads:
.275 Rimless (Rigby): 140 gr bullet, 43 gr. N.C., 2750 fps from 28” barrel, pressure 18 tons/sq.in.
7 mm (.276) Mauser: 140 gr bullet, 48 gr. N.C., 2900 fps from 29“ barrel, 18.5 tons/sq.in.
7 mm Rigby Magnum: 140 gr bullet, 40 gr N.C., 2675 fps from 29.5” barrel, 17 tons/sq.in. (maybe Rigby’s rimmed version?)
The .275 Rigby is listed with a lower velocity and pressure than the 7x57 Mauser. A bit confusing, but it matches the now CIP maximum pressures, 3900 bar for the 7x57 and only 3200 bar for the .275 H.V. Rigby.
Comparable German 1930s 7x57 loads were:
DWM: 139 gr bullet, 49 gr R2 powder, 2697 fps, 2800 atm, 26” barrel
RWS: 139 gr bullet, 49.5 gr R2, 2776 fps, 2850 atm pressure, 26” barrel
So the .275 Rigby 140 gr “High Velocity” load is a far cry from a “hot loaded 7x57”. I would expect 2600 to 2700 fps from a 24” barrel.