Well, I'm almost loath to post again, but here goes:

"Those rifles which carry bullets heavier than 400gr. and fulfill other conditions might be called Magnum Express".

So just how much weight should be accorded to Rigby's contemporary view?

At the time of his response to Stonehenge (1882?), there were only two cartridges formally carrying the name "Magnum Express". They were the .500/.450 x 3 1/4 and the .577/.500 x 3 1/8 (check Hoyem to verify this). One of the two (the latter) carried a bullet over 400 grains, the other was loaded with a 325gr or 365gr bullet. So perhaps Rigby was half right??? (...or half wrong!). Put simply, his bullet weight definition was not supported by the cartridge nomenclature of the day (or since!).

Whether or not someone is 'naive' for believing a published opinion over the facts is not my call.

In a more modern context, Burrard has something to add to the Magnum definition. In his fourth edition of 'Notes on Sporting Rifles' published in the 1950s, he encourages us to ignore the old black-powder definitions, since we all now live in the nitro age, and goes on to define a Magnum cartridge as one which achieves a muzzle velocity of over 2500 fps in at least one of its factory loadings.

Admittedly, its yet another British definition, against which the .458 still doesn't qualify!!

Regarding the champagne bottle definition, I certainly didn't pluck it out of the air. To quote a prominent poster on this board, "I remember reading this years ago, 'somewhere'!", "Unfortunately, I can't remember the original reference", "Now, all I've got to do is remember where I read that!", "Some years back, I read an article...", "Somewhere, I seem to recall.... Can't remember whether it was Gough Thomas or Burrard?", "Some day when I've got nothing better to do for a couple of weeks, I might try and dig it out." When I do come across it again I'll certainly post it here.

...and just to add a little to AlaskanPH's original query, no-one has yet mentioned that the 'Nitro' in 'Nitro Express' comes from the two main active components of the then-new double-based smokeless powder (specifically cordite), being nitro-cellulose and nitro-glycerine.