Cause of temperature related POI shift?

[gpr]

I noticed this winter that I was getting a large POI shift based on the temperature of the gun. I did some testing, and when I would leave my gun outside to get cold the POI would shift down a little over 1..5”. I wasn’t sure if it was from slower ammo speeds, or the gun. So I tried a cold gun with warm ammo and cold ammo with warm gun. The POI shift followed the guns temperature change.

I’m wondering what causes this shift? Is the temp change flexing the stock? Maybe the scope expands and contracts at a different rate compared to the gun?

Do all guns do this? I tested this on two different bolt rifles and both had similar results with large POI shifts due to temperature. It is very frustrating when hunting as I am not confident in the rifle at distance. When shooting during the summer time the POI never changes and is consistent and repeatable. Really the only solution I had was site the gun in at the temp I’m shooting it at, but that can change so I would like to get to the root of the cause.

Any ideas or recommendations on what causes this? BTW the rifles I tried it on one had a laminated stock that is bedded and one has a wood stock that is also bedded and both are free floated.

 

[hamstur]

Based on your description of the rifles, sounds like they are thinner contour hunting rifles? More to do with the physics relations of metal and temperature. Think about what happens when the barrel gets hot and starts to string and shoot inaccurately.

- Heat causes steel to become more "ductile" ... the steel barrel is more prone to whip around and extreme stress would cause the metal to warp
- Inversely, cold temperatures cause steel to become more "brittle" ... the steel barrel is more stiff and extreme stress would cause the metal to crack/fracture

The way your rifle happens to exist, the harmonics of the (stiffer) cold barrel isn't whipping as much so it's resulting in a lower POI. After a few shots when the barrel is at normal temperature, then POI is back to what you expect on a 75F day.

One way to visualize the effect is take 2 rubber bands and cut them. Put one in the freezer laid flat until it freezes. If you hold the room temp one between your fingers and shake it, it whips around like crazy. Do the same with the frozen one and it will be stiff like a twig and might even break in half. The same happens with steel but because metal is so naturally stiff, it's harder to visualize. Or ... think about T2 Judgment Day when they froze the T1000, Arnold shot it and it shattered into pieces.

 

[gpr]

These are thinner barrels since i use them for hunting. On a gun setup with heavy varmint or heavier profile barrels is this issue still prevalent? maybe just not as much? Are stainless barrels less prone to temperature related POI shift since it is stronger and stiffer to start with?

I have been debating on barreling them, but they both shoot so well that i haven't seen the need to do so yet.

 

[hamstur]

On heavier profiles the issue can still exist but far less pronounced. Visual comparison I'll throw out there is think about lumber:

- Thicker of same length is stiffer ... a 2x4 is much stiffer than a 3/4" floor trim
- Short of same thickness is stiffer ... an unsupported 12ft length of 2x4 will bow, but a 4ft length of 2x4 will not

The flip-side of thicker barrels is once they get hot, they take longer to cool off.

If you Google up "rifle cold bore shot" you will find a variant POI on the first shot is very common. I remember long long ago, Lowlight debated that cold bore POI issues were due to imperfectly built rifle or cold shooter.