Re: 1 in 12 twist 6.5 bullet weights
Unless the barrel is already made I would reconsider it. I had a 264 WM long ago with a 1-12. I used the 120 Sierra SPT which is a short bullet. The rifle was very accurate and really blew up the pests at normal ranges. That bullet really does not have the best Ci however.
You will need a faster twist for bullets like the 107 MK. That's a very long bullet!
The 264 WM that I have now is the standard twist. I suggest that you get that.