Cold weather shooting

[jeffro]

Im wondering if any of you have ever seen this. We live in AZ, so it is usually warm. 75-85 degF. My brother was shooing a 300WM. The load he was shooting is sighted in, and is shooting 3.5 to 4.0 inch groups at 600yrds consistantly. When we went out to shoot today, it was 40degF. The first round out of the gun hit where is was supposed to hit. That round was warm because it just came out of the box in the truck. The next 2 rounds were cold, and they hit 12inchs (2MOA) low. We couldnt figure out what would have caused this. I have heard about snipers keeping rounds in thier pockets to keep the rounds warm so they can have consistant round temps. We went ahead and tried this, by putting rounds in our pockets to warm them up. As the temps of the round came back up, the POI came back up to where it is supposed to be. The load he was using was: 72.5 H4831SC, Fed 215, 208Amax, 2866fps. We thought that the Hodgdon Extreme powders would be better than this, as I have never seen this with any other powders we use. What are you thought on this.

Thanks for your help, and ideas.

 

[Mikecr]

Cold powder and barrel producing lower velocity.
Nothing unusual here.

 

[jeffro]

I know that cold powder and barrel produce lower velocity, but I didnt think it would make a 2 MOA difference at only 600yrds. I have shot in the cold weather before, it does make quite a big diference, but it didnt make that much difference. The Vel at 75degF was 2866fps. For the dope to work out for this drop, the Vel would have to have been around 2670fps. Almost 200fps less with a 35degF change in temp.

 

[BryanLitz]

Does the rifle have a thin barrel?

 

[jeffro]

.660 Muzzle dia. Magnum Sporter

 

[BryanLitz]

.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.

-Bryan

 

[jeffro]

Thank you very much. You are always very helpful. I will look into this, but it sounds like this is the problem we are dealing with.

Thanks again.