The only caliber I own that is sort of off the normal (AKA not common on a walmart shelf) is 6.8 SPC. I use it to hunt in an AR-15 platform with a little more power. Honestly for ammo it's hard to beat 308. Tons of choices and lots of cheap surplus. That's why I also chose 7.62x39 over 300 Blackout. It's gonna be tough to find 6.5 ammo for less than 80c/round. But if you're okay with that, I've heard it's a good performing round at the long range end of things.