I heard so much of the .280-is-superior back in the 1980s that, as conscientious gun writer, I had to find out. Had a custom "mountain rifle" built on a Remington 78 action(exactly the same as a 700 but a cheaper, birch-stocked version) by the late Dave Gentry, which weighed under 7 pounds with scope--partly because Dave was a .280 fan. He claimed it shot a lot more accurately than the .270--but the 78 was originally a .270, which shot extremely well with most loads.
Dave's custom did not shot quite as well, because he used a lot thinner barrel--but with the right loads it shot well under an inch. Used it as my primary "light" North American rifle for around a decade. It was a very lucky rifle (which I tend to believe in), and among other animals used it to take my biggest mule deer ever, both in antler and body, and one of my two biggest caribou. But have plenty of experience with the .270, both my own and that of others including Eileen (who used the .270 as her ONLY big game cartridge for over a decade) to know every one of those animals would have been taken with similar results with a .270.
Some argue that the .280 has an advantage at longer ranges, due to the standard 1-9 twist and, heavier, higher-BC bullets. But out to around 400 yards (the realistic range when I was using the .280 a lot) I could never could find any difference. That heavier/high-BC advantage is being reduced lately with new .270 bullets, and tighter rifling twists.
But most of us aren't concerned with that. In my experience (which includes not just the Gentry .280, but others--and plenty of .270s) there's zip difference in the field. However, you can buy .270 factory ammo just about anywhere, which isn't true of the .280. That has been a factor even in Montana, where a friend who was long-time .270 fan switched to the .280 for couple years. He even outfitted his family with .280s--and for various reasons they ran out of ammo on an eastern Montana pronghorn hunt. There wasn't any .280 factory ammo locally--despite the 90s still being part of the ".280 is superior" era..... He went back to the .270, and killed a B&C bighorn ram with one this fall.
