temps vs zero

[coyote1758]

how does the outside temp change zero on your gun.I zeroed in at 78 and check again ay 32 and was off 6.5 inches.Can the temps do this?help please

[MontanaRifleman]

That seems like an extreme difference, but some powders are very sensitive to temp changes. I have heard as much as 2 fps pre degree of temp with some. The difference in velocity wouldn't change the POI that much, but a difference in pressure might affect your barrel's harmonics.

You might have something else going on too. How often have you checked your zereo and how consistant has it been?

Mark

[coyote1758]

I shot most of the summer and was shooting out to 650 yrds and not changing the zero. Have not checked it with the temp drop but was surprissed at the differance. The scope is a nightforce 5.5-22x56 and the gun is a savage pred.10 in 22-250 and the powder is hogden 4895.Just been reloading about 8 mos.Seems that the gun lights the poder but not sure I like the change in zero .Should I change powder or just check zero more offen?

[demented]

Shouldn't be your powder, not that radical zero shift. The cold HAS to be affecting your mounts, barrel.....something. Have you tried leaving your rifle inside your vehicle at the range, keeping it warm until you have everything ready to go, firing before the cold has time to creep in? This would at least tell you if your scope has lost zero providing its not still shooting poa.

[MontanaRifleman]

I agree with demented, H4895 is supposed to be stable. It might change a little, but I wouldn't think that much. Have you chrony'd your shots?

My guess is something else is going on.

[Bart B]

I've shot 1000 yard matches on one range in temperatures from 25 to almost 100 degrees. With my .308 Win., there's about a 4 MOA difference in sight elevation to zero the same load. But my .30-.338 only needed about 3 MOA change. It's probably a combination of powder temperature (low temps lowers velocity) and air temperature (lower causes bullets to slow down more).

If I keep a round in a hot chamber for more than 30 seconds, it'll shoot the bullet faster. About a 1/4th MOA come down in sight elevation's needed for every 30 seconds a .30-.338 round's in that hot chamber (every 20 seconds for a .308 Win.). I've made as many as six or seven 1/4 MOA come downs during the 3 to 4 minutes that rounds in the barrel, shot to call, then came back up that amount shooting the next round within 20 seconds of chambering it; the shot went to call.

So it's my experience that both powder and atmospheric temperature makes a difference. If you know what it is and how to correct for it, you'll do well at the longer ranges. It ain't rocket science; just grade school physics. Go do some tests with your rifle and its ammo then you'll know how much to correct.

[coyote1758]

Thanks for the help.After shooting this summer I learned alot but still have a long way to go.This is the first time I have practice at this range,150-650 yrds.It is fun and you learn alot about your gun.Going to shoot today and I am going to start by leaving the gun outside to cool first to ck my cold bore shot and work from there.Thanks again.