Re: primers for low extreme spread in 264
I don't shoot that cartridge or powder. But, Fed gm215m is a good and consistent primer.
Assuming you've already zeroed in on an accurate load to begin with.
Most often, I find that high ES/SD is related to inconsistent brass prep. ...specifically neck wall thickness variation (turn and/or use bushing/collet die)
Also, chrony readings can be fooled with lots of muzzle blast. Move the chrony out farther if possible. Verify actual groups at long range.