260 is not considered a barrel burner. Won't have 5000+ round life like 308, but is a lot better than the likes of 6.5-284, should get 2500-3000.
If you plan to use factory ammo, you best choice is definitely 308, but if you plan to reload everything then both 6.5x55 and 260 have readily available components. You can actually get Normal training ammo for the 6.5x55, it's not loaded very hot, but it's pretty reasonably priced and gets good reviews accuracy wise.
I've never bought factory ammo for my 260, and hope I never have to $$$$$$$
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