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.

 

[Bart B]

Why are you going to cool down your rifle to check you first cold-bore shot?

If the first shot from a cold bore's not where the rest of them go, there's something not quite right with your rifle. Through 600 yards, the first shot from a cold bore should be within 1/3rd MOA of where 5 to 20 or more shots center up afterwords.

 

[Pdvdh]

Why are you going to cool down your rifle to check you first cold-bore shot?

If the first shot from a cold bore's not where the rest of them go, there's something not quite right with your rifle. Through 600 yards, the first shot from a cold bore should be within 1/3rd MOA of where 5 to 20 or more shots center up afterwords.

I would guess because he's trying to troubleshoot his rifle. Also, he may not be so much interested in knowing how his rifle will group a 5 or 10 shot string as the barrel heats up, as he is in knowing where his first shots from his hunting rifle hit with a cool barrel on the first shots. Ideally a gun wouldn't move point of impact from cool barrel to a heated barrel. But the thinner tubed versions often do start to enlarge the groups as they heat up. So I also cool my gun barrel down in the effort to learn how well the gun will group with a developed load with a cool barrel, rather than a barrel that starts out cool and ends up at 100F. In fact when there's snow outside I'll grab a handful and run it along the outside of the barrel from the forend on out to the muzzle to reduce the length of time I'm twittling my thumbs waiting for the barrel to cool down.

 

[Kevin Thomas]

Hey Guys,

Since I haven't seen anyone else mention this I thought I might chime in with a thought. Since we're talking about a major change in POI between 32 degrees and 78 degrees, everyone seems to have focused on the burn rate, ignition and velocity of the load itself, while leaving the temp itself out of the conversation. I'm sure you guys know this, but cold dry air is much denser than warmer more humid air. At these sorts of distances, it makes a significant difference and has to be taken into account. My guess here would be 1) atmospheric differences first, 2) powder or velocity variations second.

Kevin Thomas
Lapua USA

 

[MontanaRifleman]

Kevin, that's a good point. For some reason I over looked the cal being shot which is going to be a small and low BC bullet. Running the numbers for a 55 gr bullet with a BC of.26 and an MV of 3600, the differnce in drop with temp range would be right at 6"

 

[Bart B]

Shooting a .308 with Sierra 190 HPMK's out of a 28 inch barrel at about 2600 fps to a 1000 yard target, there's a 5 MOA difference in sight settings for elevation at 6600 feet down to 600 feet. Air density get thin the higher you shoot at.

 

[Kevin Thomas]

Atmospherics can make a big difference and they can be surprising at times. I just hadn't seen anyone mention it thus far, so it seemed worthwhile to throw into the mix. Hope it helps!

Kevin Thomas
Lapua USA

 

[coyote1758]

Thanks for all the help.I went yesterday and got sighted back in, the temp was around 30 and my mpb is suppose to dead center at 257 and shot 3 shots in a 1/2 group, 1/4 from bulls eye and feel a lot better.I have learn to ck my gun more often during temp changes.I will tell untill shooting out to 650 yrds I never gave it much thught about trying to keep tight groups.I will never keep up with most the guys on this forum but it is fun to shoot that far out and am looking forward to having some more fin this summer.Again THANKS FOR ALL THE INFO IT WAS A BIG HELP.Thank chuck