Re: 1000 Yard Gun - .260 vs 6.5 Creedmoor
The 6.5x47L/6.5CMs were never intended to be used for 1kyd competition. They are under powered for it, as even the 260 with a full 5gr more capacity is still underpowered for 1kyd.
My basis for this is that you cannot reach the ~2950 node with ~140gr highest BC bullets without high pressures, and overworking of brass, compared to higher cap cartridges like a 6.5x284, or 6.5WSSM, etc.
The perfect 26cal capacity in my view is the 260AI(which matches the 26WSSM).
Ultimately, both the touted mighty mouses and the 260 are variant 26-243 wildcats, and there is nothing 'new' or unpredictable about them.
What I like about the 260AI, or other improved forms of the 260, is that they remove much of the body taper, and increase shoulder angles to prolong brass life -with rational pressures. It also makes reloading way easier, as far less sizing is needed, and so less brass movement/trimming of it.
I don't see a reason to go with 600yd bullets(123-130gr) for longer shooting, just to reach a common accuracy node with a cartridge that's too small to do it otherwise.
So I say go atleast 260, improved or not.