Is a .25-284 close enough ? I've hunted extensively with two .280's.
Based on experience with other rounds and enough long range shooting to understand the difference, the 6.5-284 would have the edge at ranges over 400-500 yds. That's because the heavier 130-140 gr. bullets for it have very high tested BC, on the order of .600, thus they drift less in the wind. The .280 can't push anything with a tested BC that high nearly as fast. That also means the bullets for the 6.5-284 will expand further out.
I'd like to see the guy that can tell the difference between the wound channel made by a .264 bullet and the same class of bullet with a .284 diameter.
As big a fan as I am of the .280, I have to admit that the factory ammo for it doesn't appear to loaded to the round's full potential. If I were stuck with factory ammo, I'd buy a .270 instead of a .280.
In a long action rifle, you can load the long 6.5 bullets out where the base of the bullet isn't seated below the case's neck and shoulder junction. This tends to make the rifle more accurate.
So, if you are doing a dedicated long range rifle, I'd say the 6.5-284. If you are after a regular sporter for 400+ yd. shooting, I would bother with the 6.5-284. E
