Question for the guru's. My .280 load gives 1846 FPS with 1211 ft lbs at 450 yards. I'm shooting a 160 gr partition at 2635 FPS. My 6.5x284 load gives 2092 FPS with 1360 ft lbs at 450 yards. I'm shooting. 140 gr accubond at 2873 FPS. Since the 6.5x284 has more velocity and energy should I use this for my elk hunt? I know some of you will say I need a larger caliper but this is what I have to work with. Please give me your opinions. Thanks!
I think that you should reconsider your starting points and redo your load development for the 280, especially. 2635fps is somewhat anemic for a 280 with 160 grain bullets. I would not recommend hotrodding a 160gn to 3000 fps, but 2900fps is often doable according to Nosler and 2800fps should be easy to obtain with an accurate load. Your 6.5-284 is closer to its average hunting load levels. In effect, the comparison was an "apples and oranges" affair.
What to do?
Well, for one, if you are seriously planning on shots at 450 yards, you might try equivalent bullets and a slightly better BC for the 280. The .284" Partition BC isn't bad at .475 but you are comparing it with a bullet with a BC of .509. If you stayed within the same company you could shoot a .284" 160gn Accubond with a .531 BC.
Problem solved.
You could also check out specialty bullets like Barnes 145gnLRX with .486 BC or 168gn LRX with a 550 BC. Lehigh Controlled Chaos has an amazing 142 gn bullet with comparable BC to the Partition .477 but exceptional terminal effects and Hammer Hunter has some sleeker bullets with similar effects. I tend toward monolithics because I deem deep penetration as a primary consideration after accuracy, and penetration needs to be guaranteed up close, too.
In sum, the 280 has a paper advantage in both diameter and energy, and is equivalent/comparable in velocity. However, like most big game hunting in the US, the ballistic advantage of the 7mm over the 6.5mm is moot and would only apply to some marginal situations.
The deciding factor should start with accuracy. Which rifle is more accurate? If the they both consistently shoot under <1 MOA, then go with the 280. Otherwise go with the rifle that is consistently under 1 MOA with the bullet/load chosen. Then you will have the best chance of tasting that elk standing 450 yards away.