I have rebarreled a couple of 300 win mags into 6.5 rem mags, 1 for me and then a friend wanted one.
Mine was a Browning BBR 300 win mag with a short detachable magazine so the closest I could load to the lands was .250" + (which irritates the hell out of me) so velocity was low for a 300 win mag. With the 6.5 rem mag I had the smith set the lands so I can seat those long 6.5 bullets way out there and it still fits the mag
On mine I put a Douglass XX barrel finished at 26" with a 1 in 8 with #4 contour. It has a McMillan stock and a Conquest 4.5x14x44 RapidZ 800
It will shoot almost anything well, generally 140 gr bullets at 3100 fps and 130 gr at 3200 fps. For the ballistic coefficient and to match the RapidZ 800 reticle I settled on 130 gr Scirrocco II's at 3200 fps with RL17
I've had a couple 264 win mags and both were a bitch to load for and problematic with dies and cases. The 6.5 rem mag has plenty of dies and brass is available, Remington but it is good with some work. The caliber is easy to load for. You don't need a super slow powder like the 264 win mag.