Originally Posted by jeffro
.660 Muzzle dia. Magnum Sporter
OK, that's not really thin
, but not quite heavy either. Here's what I'm thinking...
Barrels whip around in harmonic patterns that are proportional to the bullet weight, velocity, cartridge capacity, barrel contour, etc. Thinner barrels will 'whip' more than heavier barrels. The location of the muzzle in its vibration pattern when the bullet exits has a big effect on POI. The interplay between muzzle velocity variation and the barrel harmonics can lead to some favorable or unfavorable interactions.
If the slower than average bullets exit when the muzzle is pointed up (or on an upswing), there is a favorable compensating effect. Rather than that slow round hitting low, the barrel harmonics can compensate by releasing it high.
However, if the harmonics happen to work out so that the bullet exits when the muzzle is pointed down (or on a downswing) for the slow rounds, then you can have a compounding bad effect that would make the slow rounds hit even lower than they would just from being slow.
In summary, this is what I'm suggesting may be your problem. The cold results in lower velocity shots, but not enough lower to hit 2 MOA low at 600 yards. However, the lower velocity combined with unfavorable barrel harmonics just might add up to shift your POI by the amount you're observing.
How to solve the problem? Assuming I'm right, the easiest way is to keep the rounds warm (in a pocket close to your body as you described) so they achieve the average MV that you're counting on. More serious solutions would be to try different powders/charges that result in favorable compensation rather than unfavorable. You can test for this by doing a ladder test at long range.
Due to your combination of heavy bullets from a magnum cartridge and a non-heavy barrel, I think the above explanation about barrel whip is a possibility. Of course I can't say for sure.
Good luck, let us know if you make any conclusions with this.