QUESTION-temperature POI shift

chopper 1

chopper 1

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I was sighted in for summer and at +6 was dead on (100yds). Shot again same way at -9 degrees and was almost 8 inches higher. Is that possible ? Or was it some other factor.I can surmise colder air is thinner making the bullet path higher - but 8 inches? :eek: any thoughts ?
 
Cold air is actually denser/heavier so if anything you should be lower. Something else has changed. You at the same range or a different one, since a change in altitude will have the greatest influence followed by changes in temperature on BC of a bullet or maybe a different powder lot. This example has a 15 degree temp change and that is only worth .1" on the target at 100 yds.
 
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Ballpark figure:

count 1 MOA change for every 20 degree temp change.

lower temp... lower POI and vice-versa.

8 inch change......:confused:

check other variables in shooting system.

BB
 
Hi

Chronographs are getting pretty cheap these days. They are a *big* help working out these kinds of problems.

Of course in this case a time machine would also be required :D

Bob
 
Was the group 8 inches higher or just the first shot?
Some powders are more temperature sensitive, but an 8 inch shift is huge.
Was the bore patched out (dry) before you started shooting? You don't mention what rifle. If it's a bolt rifle and you were a little generous with the gun oil last time you cleaned it, there could be oil or cleaning solvents between the action and stock. Are the bedding screws tight? Is the rifle bedded? Scope bases and rings tight? Lot of things could be the cause.
 
Hi

How is the rifle bedded? If you have contact between the stock and barrel, all sorts of interesting things can happen. That's true both with plastic stocks and with wood ones.

Bob
 
maynard said:
Was the group 8 inches higher or just the first shot?
Some powders are more temperature sensitive, but an 8 inch shift is huge.
Was the bore patched out (dry) before you started shooting? You don't mention what rifle. If it's a bolt rifle and you were a little generous with the gun oil last time you cleaned it, there could be oil or cleaning solvents between the action and stock. Are the bedding screws tight? Is the rifle bedded? Scope bases and rings tight? Lot of things could be the cause.
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no wood, in fact no stock - no oil in barrel - rested on receiver both times , all rounds - 30 or so - were 8 inches High - suspected my aim point micro but cant see how - possibly my muzzle brake which I removed and re attached - the only other difference was that I cleaned the rifle but after 60 or 70 - no change so I re zeroed the sight - now I'm waiting to see if the groups moves down as it gets dirtier LOL :confused:
 
poi shift

pa guns said:
Hi

What kind of rifle are we talking about here?

Bob
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cz 858 -
loadedxmaseve009.jpg
 
Hi

Ok, *that* makes a difference.

I would definitely take a look at the gas tube. You may have a situation where it's putting to much pressure on the barrel. That will lead to very interesting behavior. You will get one point of impact this time and a completely different one next time.

Bob
 
poi shift

pa guns said:
Hi

Ok, *that* makes a difference.

I would definitely take a look at the gas tube. You may have a situation where it's putting to much pressure on the barrel. That will lead to very interesting behavior. You will get one point of impact this time and a completely different one next time.

Bob
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velly intelesting ! Now that I think of it the forend locks on with a screw at the bottom putting pressure on the front of the receiver forcing the forend tight against the front barrel band gas block assembly - maybe I re-installed it tighter or looser than last time - I will now shoot groups with varying degrees of tension and see wha hoppens -- THANKS !:D
 
Hi

It's a common issue with all of the weapons based on an AK-47 gas system.

You don't want things to loose - bad things can happen. You also don't want them super tight - the tension is never the same. It's a compromise that has to be slowly worked out. Taking metal off is a *lot* easier than putting it back on. Essentially you are free floating the barrel from the gas tube, but they must stay in contact.

Bob
 
poi shift

pa guns said:
Hi

Ok, *that* makes a difference.

I would definitely take a look at the gas tube. You may have a situation where it's putting to much pressure on the barrel. That will lead to very interesting behavior. You will get one point of impact this time and a completely different one next time.

Bob
Click to expand...

Thanks Bob - you were absolutely right ! -I loosened the screw , the groups were low , I tightened it and the groups were high (all centered though).Seemed to shoot consistently when the screw was tighter {need to shoot more groups to find out} but the standard gun was designed with little pressure from the forend on the gas block ferrule so MORE TESTING to see which way it will shoot better. Thanks again for the info --:D
 
Hi

Definitely experiment with the rifle you have. The free float approach only is necessary if you have a light weight gas tube and a really heavy / nasty chunk of wood attached to it.

Bob
 
chopper 1 said:
- possibly my muzzle brake which I removed and re attached - the only other difference was that I cleaned the rifle but after 60 or 70 - no change so I re zeroed the sight - now I'm waiting to see if the groups moves down as it gets dirtier LOL :confused:
Click to expand...


Muzzle brakes move the point of impact. Also you say you rezeroed the rifle? That will change things specially if you rezeroed with stock off?

NOt really the same conditions is it?

NormB
 
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