change of POI due to differential expansion rates rifle-scope ?

P

pierre

Guest
let's start with an example :
it's winter time ,freezing ,your 1000y zero is perfect no wind
next time you shoot is summer .Again no wind ,but temperature is 80F
You knowyour shooting tables ,played with your software ,you can then adjust your sights accordingly
you shoot and...there is a difference
Could part of the difference be that rifle receiver /riflescope /mounts expand differently and this changes the point of impact ?
If yes ,what have you noticed ?shooter higher ? lower? to the side ? depends on scopes /rules of thumbs?
 
kinda-probably-maby-hypothetically-

thats my answer and Im stikin to it!!!....certainly everything expands and contracts (coefficient of expansion for all materials) with temp changes....but guess what ..you dont have to wait till winter/summer to see this .....just the time between targets at a match the temp./sunlite etc. can/will change and move you around on the paper a noticeable ammount!!!....if the temp. humidity. altitude. powder charge. are different ,,,then it will make a "differance"......Roger
 
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I've got it!!!!!!!!!!

Because you are sitting at the bench in your shorts versus in your snowmobile suit the angle that your your eyeball is looking in the scope is different!!!!!
 
Pierre,
That is an easy test. Keep your rifle indoors in the A/C, setup a temporary shooting platform in your backyard (portable bench, patio, anything). Get it all setup and bring your rifle out and set it in the bags just like you were going to shoot. I pointed my rifle at one of the rivet heads in my shed about 50 yards away. At the higher power on my scopes I could 1/4 the rivet head. Leave it sit and bake in the sun for 1/2hr or 1hr whatever you feel like. The steel action or barrel should probably be really hot to the touch at this point. At least it was with the mid-summer Carolinia sun when I did this.

I didn't see any "creeping" of the point of aim that I could tell. I did peek through the scope at various points while the rifles were baking in the sun. And this readical of a temp change from A/C temp inside to the whole rifle being hot to the touch is more than you will ever see during live firing in competition.

I did this test for different reasons, but the result is the same no matter what your trying to learn from it.

The temp, humidy, and baro pressure changes is going to change your internal ballistcs more than anything that might be affected externally from my experience.

Steve
 
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Steve Shelp said:
Pierre,
That is an easy test. Keep your rifle indoors in the A/C, setup a temporary shooting platform in your backyard (portable bench, patio, anything). Get it all setup and bring your rifle out and set it in the bags just like you were going to shoot. I pointed my rifle at one of the rivet heads in my shed about 50 yards away. At the higher power on my scopes I could 1/4 the rivet head. Leave it sit and bake in the sun for 1/2hr or 1hr whatever you feel like. The steel action or barrel should probably be really hot to the touch at this point. At least it was with the mid-summer Carolinia sun when I did this.

I didn't see any "creeping" of the point of aim that I could tell. I did peek through the scope at various points while the rifles were baking in the sun. And this readical of a temp change from A/C temp inside to the whole rifle being hot to the touch is more than you will ever see during live firing in competition.

I did this test for different reasons, but the result is the same no matter what your trying to learn from it.

The temp, humidy, and baro pressure changes is going to change your internal ballistcs more than anything that might be affected externally from my experience.

Steve
Click to expand...

Nice idea!!!!!!!!!
 
been there don it...

not with a 1000yd gun...but with an unlimited rail gun for short range use ..100-300....it was bak in the late 80's ..but the theroy hasnt changed....it was a hot summer day ..aprox 90 F...and my rifle was in the house at 75 F...the rifle was sighted in from the previous days shooting ...also 90+F...I sat the gun up on the bench and got er dialed in ....the first shot was aprox 4 inches low ...then then next aprox 1" then it settled down and shot several rite in the same spot ....the bbl had warmed up....the ammo didnt have time to....I had the gun set up and ready before I brought the ammo out of the house ...and the set up time took aprox...2min....so it was mostly metal changing and expanding not the ammo....I remember the gun even got a lil wet looking from the condensate on the cold metal parts.....Roger
 
Change of POI

Prairie dog hunting we found we had to come down a couple of clicks in the afternoon heat. Started shooting off a bench with a golf umbrella shading the gun (and me) from the sun and no two clicks in the afternoons.
 
I suspect that that the effect of differential expansions would be lost in the noise of more critical effects.

I shoot a belly sport at 1000, 1100 & 1200 yards. We shoot from 10 to 12 day one & back the other way day two. On one range where there is a lot of difference between day & night temperatures, humidity & all that stuff, we usually need 2MOA more elevation the morining of day two than we used the evening of day one. Morning has no humidity, cold ammo, little mirage compared with the evening.
 
Why don't you play with loosening and then tightening one scope ring cap, to release any longitudinal tension or compression on the scope, and see if the POI returns to the pre-heating position?
 
do you know if there has already been something written on that subject ,even marketing litterature by scope manufacturers?
just tu ubnderstand a bit of the theory behind that
 
Pierre,

As my friend says "you are leaping over mouse turds" with this subject. The coefficient of thermal expansion for steel, be it the action or the scope is going to be the same. There might be variations due to each alloy, but they will be very, very small. Once everything is allowed to come to the same temperature it will not matter.

Jeff in OH
 
Hi Jeff
thanks for your comment
I was just trying to understand how the movement in temperature could differentially affect very different items like :
glass,seals,screws,erector tube,nylon bushes ...
and if different scope manufacturers have approached this question
no big industrial project behind this thought process,just for fun and chat !!
Pierre
 
pierre said:
Hi Jeff
thanks for your comment
I was just trying to understand how the movement in temperature could differentially affect very different items like :
glass,seals,screws,erector tube,nylon bushes ...
and if different scope manufacturers have approached this question
no big industrial project behind this thought process,just for fun and chat !!
Pierre
Click to expand...

Pierre,

I am sure the scopes are made to withstand the extremes of the temps. I do not know the construction inside of the scope to be able to comment. This is why I buy Nightforce and Leupold almost exclusively. Lifetime warranty ;).

Hope you find your answer.

Jeff
 
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