Thought I'd toss some comparative data on the end of this thread, just for the record.

Ballistics and load data from the 1926 ICI catalogue:

Cartridge ---------- Powder- Bullet- Pressure- MV(fps)- ME(ftlbs)

.500 BPE ----------- 136gr - 340gr -- 11 tons - 1925 -- 2800
.500 NforB --------- 55grC - 440gr - <10 tons - 1900 -- 3530

.577/.500 #2 BPE --- 130gr - 340gr -- 10 tons - 1850 -- 2590
.577/.500 #2 NforB - 53grC - 440gr - <10 tons - 1675 -- 2743

.577/.500 Mag BPE -- 164gr - 440gr -- 11 tons - 1875 -- 3440
.577/.500 Mag NforB- 60grC - 440gr - <10 tons - 1725 -- 2910

You will note some interesting things here, such as 60gr cordite in the .577/.500 Magnum generates only 1725 fps, whereas in the .500 straight-case the lesser charge of 55gr cordite generates 1900 fps, both with 440gr projectiles. As all modern reloaders of these cartridges know, the larger-volume case is more of a handicap than a help when loading smokeless!

Looking at the Muzzle Energy figures:
The clear winner in BPE loadings is the .577/.500 Magnum.
The clear winner with NforB loads is the .500 3-inch straight-case.

Sadly the .500 No.2 lags behind in every respect.
It is a charismatic cartridge though, and I dearly love the Rodda so-chambered in my collection.

Of course, today it is possible to load these cartridges up or down at will, especially in a single-shot, but it is likely that most quality doubles will regulate at or near the ballistics published in the catalogues of the day. Most of mine do!