Re: cold temperature and high b.c. bullets
It's a matter of twist and stability, not necessarily BC.
For the 115 VLD's we recommend a 1:7" twist to insure stability in all conditions. I've shot them out of a 1:8" twist 6mmBR and had them work just fine, in the warm summer air. I suspect if I tried it this time of year, I'd have the same stability/accuracy problems you're seeing.
Stability depends on a lot of things, air density being one of them. Colder air is more dense, so it's harder for the bullet to stabilize in it, even if it's the same muzzle velocity and twist rate that works well in warmer (less dense air) conditions.
It may <span style="font-style: italic">seem</span> to be related to high BC bullets because they usually have the most strict twist requirements. Shorter lighter bullets are comfortably stable in all conditions, but the long heavy bullets need a little more care and attention to conditions.
Hope this helps,
-Bryan