Which Caliber

[6mm Remington]

280 AI is a fantastic cartridge with stellar ballistics, excellent bullets for it, and it won't beat you up shooting it. No muzzle break required!!!

 

[CV335]

6.5 PRC. Awesome caliber and great ballistics. Fun to shoot!

 

[Cayden]

300 win mag will destroy anything that walks in north amearica

 

[TREE ‘EM]

.270 Winchester has been doing everything you’re asking for a hundred years and keeps getting better with age.
Cheap decent quality factory ammo that can be found almost anywhere should the need arise.

Component availability pretty much everywhere.

Newer powders and bullets make the .270 a much bigger gun than it used to be. 150 grain Accubonds over Reloader 26 are breaking 3100fps from my 22” barrel. I just bought some ABLR 165’s to try in my 8 twist and 8.5 twist customs.

And there’s where we come to the current limitations of most .270’s, twist rate. Most manufactures are still stuck on 10 twist or worse yet 11 twist barrels which are intended for 140 and 130 grain bullets respectively.

To my knowledge, Barrett was the only manufacturer to offer a 9 twist which is golden for 150 grain bullets. Mine will keep half MOA with 150 grain partitions at 3100+- FPS

All that said, the 6.5X55 or 7X57 is classic cool and in the same camp of being much bigger than they used to be with modern components.

 

[rootacres]

With you already having the 300 PRC I went with the 270 WSM. IMO its one of the more underrated cartridges. The bullet options continue to become more expansive. I built a 7mm WSM, another very underrated cartridge. Both sit on a short action and have tremendous ballistic when paired with the right bullet. You'll probably use the 300 PRC for game elk size and larger. The 7mm WSM or 270 WSM would be great for deer size critters and smaller.

Plus, with all the rage on the 6.5 PRC its fun to do something a little different.

 

[EmperorMA]

Off your list I like the 6.5x284 followed by the 6.5 PRC. Both can serve as a backup to a .300 PRC and both can take its place if you get tired of shooting that blunderbuss in a lightweight rifle.

If you truly plan on keeping that .300 PRC, then I would honestly just get a 6.5 Creedmoor. It will have by far better factory ammo with a much wider range of factory-loaded bullets, plus it is much easier to shoot and much easier on barrels. It will easily serve as a backup to your .300 PRC out to 700 yards or a bit farther and it will be more enjoyable and economical to shoot ..... meaning you will shoot it more! I am not sure why you want "more energy" because energy doesn't kill animals, but impact velocity and bullet construction sure do. You can find bullets for the 6.5 Creedmoor that will do just about anything you want within those two parameters.

While I like the idea of a 6 Creedmoor, I just don't think it is big enough to serve as a backup to a .300 PRC. The 6.5 Creedmoor can with bullets up to 156gr.

 

[Sled]

I'd stick with the 270 win or 6.5cm simply for availability of ammo, cost, soft recoil and performance.

Fwiw, I have both the 270 win and wsm. The wsm is nice but not really that big of a gain to warrant the lack of ammo availability or increased cost over the win.