ok here is why i like the 6.5's for this build: bullet selection. I dont want to shoot any heavier than 140gr and not much lighter than 100gr. so lets say for light practice/ varmints 95gr vmax, medium game 120gr amax or nos bt, even elk or black bear under 300yds you can use 130gr berger or nosler accubond and even the 140gr berber hunting vld. 6mm highest normal bullet is 105 and in a 7mm lowest normal is 120 so thats why im kinda settled on the 6.5. .........
That changes the equation, and it was not part of your original question. "Muleys and smaller". As for the 6mm's, the bullet selection goes up to 115gr, but the 105 Amax has made some great kills on deer out to 600 yards. I haven't pushed these on deer size game much further, but large coyotes have tasted them @ 800.
If I had a chance at 300 yard elk while hunting deer with the 6mm, I would carry the Nosler partitions in my loops and stay away from the shoulders.
6.5mm is a great caliber though, and if elk are in the realm of possibility, then by all means.