The 7mm Mashburn Super Magnum is based on a shortened/necked-down/blownout .300 H&H case, with the case length being 2.630", according to P.O. Ackley Handbook #1. Velocity for a 160 gr bullet was around 3200FPS. There were several other calibers based upon this length.
The .300 Win. Mag. case comes-in at 2.620".
Not much difference between the 2 cases in length.
Top loads in a 7MM Weatherby w/160 grainer is approaching 3200FPS.
Top loads in a 7STW approach 3200FPS w/160 gr.
The not-so-popular "7MM Mashburn Magnum Long", different from any of the other 7s, used the .300 H&H case full-length, 2.850". Loads with a 160 gr. bullet duplicate the 7MM Weatherby Magnum velocities. Of course, powder selection back in those days was not as it is these days, so different velocities may be attained when using more modern powders.
Best of luck to ya,