I don't have a rifle for the 275 H&H, but am considering having one built. I've done a bit of research on it and if you care to hear it:
Based on the .375 H&H belted rimless, cut to 2.50" and necked down to take .287" bullets (not .284") by Holland and Holland specs. Woodleigh and Hawk both make .287" bullets. The .275 H&H, 280 Ross, .280 Halger, and 280-333 Jeffery all use this diameter bullet.
Ballistically it is identical to the 7mm Rem Mag. The shoulder is a little more forward and the shoulder a little more steeper, but the case has more taper than the Rem Mag. Introduced in 1911 or 12, I guess it was the original 2.50" Magnum case cartridge.
Depending on who actually built the rifle, they may have used a 7mm barrel blank. I guess this was fairly common in the 40's and 50's.
