It all comes down to military fashions, which spills over into hunting fashions. The Brits settled on their .303 in the 1880s and did not take note when the ideal caliber was worked out in the 1890s, from 7mm Mauser to 6.5mmx55 Scandinavian and 6.5x51 Japanese and the 6.5 Carcano. The pendulum then swung up to 8mm Mauser and a larger caliber 30/06 and WW2 stopped Britain from their plans to ditch the .303 in favour of a thinner bullet. Post WW2 the pendulum swung sideways from the .30/06 to the basically equivalent but shorter cased .308 Winchester / 7.62x51 NATO, then steeply (and maybe too far down) to the 5.56x45 NATO --- the last two choices products of the USA going it alone in their "cooperation" with their allies. We're now seeing sense prevail and the original 6.5mm / 7mm 1890s Goldilocks again prevails, being "just about right". Meanwhile deer successfully continue to fall victims to all the above rifle cartridges at normal hunting ranges.
"All will be explained" in an article on the 1950s .280 (7x43mm) British, itself vaguely similar to the less known WW1 .256 British (AKA 6.5x51SR Arisaka). Link..
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