Why? A large part is 'fashion' in the current mantra that a short fat case will always outperform an equal capacity longer thinner design. personally I think this is at best wildly overplayed, and is quite possibly complete bunk!
People will sensibly enough chose what match winners use too - so that's .284 Win, .284 Shehane, 7mm Rem SAUM and variants on the 7mmWSM, 7mm/300WSM etc theme at the moment.
Finally, Lapua makes brass that is easily reformed to the two .284 forms, and Norma makes SAUM and WSM brass, while the .280 is seen as inferior in this respect. This is a very valid argument for the .284 and short magnums, BUT ..... you can reform Lapua .30-06 brass into .280 Rem / .280AI.
Capacity wise, the standard .280 Rem and .284 Shehane are not much different and the .280AI is identical to that of the 7mm Rem SAUM, so they'll give the same performance MV wise.
The short magnums need a larger dia. faced bolt and while the .284 Win uses a standard bolt configuration, the 'straight .284' is a little underpowered in my view for getting 180s into the 2,900-2,950 MV node that 7mm shooters say is essential. The .284 Shehane would be my ideal 7mm F Class cartridge - it works, two years running GB F Class Champion, Scotsman Grant Taylor uses one and has seen off a large pack of 7mm short magnum users - but you cannot get off the shelf dies, not even as limited production 'Group D' type items, while all top die manufacturers list .280 and .280AI versions as standard. Unless Forster, Redding, or Wilson adopts the Shehane, methinks my first 7mm will be a slightly short-chambered .280AI to suit reformed Lapua .30-06.
Laurie,
York, England