No expert in this at all, but I have used a 1:8" twist .243AI Win shooting the 105gr A-Max in Colorado, which was an excellent rifle, shooting winter coyotes with ease past 600m. Very very accurate, effective chambering.
The other option is the 6mm Creedmoor, which achieves pretty much the same outcome.
There are some fast wildcat alternatives, maybe @Wingman can comment because I believe he's been looking at this in the last few days. @Kiwi Greg too, he'll be well clued up.
I would second @257weatherby on the matter of game weights - I'd be a bit skeptical of shooting deer with a 105gr 6mm past 400m. To me, past 350-400m, a fast heavy 6mm is a great varmint option but a poor deer option.
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