Poi change

[Rabbit foot]

Just been to check zero to find poi 2 inches low.
I'm sure around spring time the poi was 2 inches high so I keep putting it back on centre
It seems to happen 6 months or more apart
I was wondering if it could be due to air temperature change but it's the wrong way around it possibly, shoots high when colder and low when warmer.
Which I read up should be the other way around?
Also 2 inch deviation seems extreme for only 100 yards!?
My obvious thoughts are scope issue? But it's constantly swapping approx 2 inches high or low when it happens,
I would understand if it carries on in the same direction to suggest fault..
Windage not an issue
.243
Meopta R1
Bi pod and rear sand sock
Very still wind

Thoughts please
Thanks

 

[VSS]

Maybe you wear bulkier clothes when the weather is colder?

 

[scotch_egg]

What stock?

There could be a variation in pressure on the bipod.

 

[Rabbit foot]

What stock?

There could be a variation in pressure on the bipod.

Funny you should ask actually as it's the howa over mould.....
I would expect something more inconsistent than that tho with the stock
All shots taken usually in still conditions all rested in the sma e place no downward pressure on barrel
100g federal powa shock too

 

[Smellydog]

We use to have to rezero every spring and autumn.
It's to do with the position of the sun and moon phases.

 

[Jagged77]

Have you left the barrel dirty or given it a thorough clean? A clean barrel can take some settling back in.

 

[scotch_egg]

Funny you should ask actually as it's the howa over mould.....
I would expect something more inconsistent than that tho with the stock
All shots taken usually in still conditions all rested in the sma e place no downward pressure on barrel
100g federal powa shock too

Enough said!

The overmould stocks are terrible. It could be as much as leaning at a different angle with those stocks. They are a great rifle let down by that stock. It doesn’t take much pressure to move POI.

 

[Rabbit foot]

I would expect a more sporadic inconsistency if it was the stock but I will keep it in mind
I have had this set up a good few years now
I have a weatherby 308 that is interchangable with the howa and it has a wooden stock so perhaps I should try that for a bit but it doesn't float so could end up with same problem no doubt

 

[Rabbit foot]

We use to have to rezero every spring and autumn.
It's to do with the position of the sun and moon phases.

Struggling to tell if your serious or not

 

[HonestJohn]

What scope and age? And is the vertical at the far end of its adjustment range? Asking as the springs can tire and give, which is especially pronounced at the end of adjustment ranges due to one. Wind very relaxed and one being very compressed

 

[Heym SR20]

Powder burn rates will vary with ambient temperature so you will get a difference between winter and summer. Depends on temperatures and powder type but I recall it can be 1 fps per degree Fahrenheit.

In winter air is also a lot denser than in summer time so more air resistance to flight of bullet - one of the reasons most ballistic Apps include temperature and air pressure as variables.

 

[Rabbit foot]

What scope and age? And is the vertical at the far end of its adjustment range? Asking as the springs can tire and give, which is especially pronounced at the end of adjustment ranges due to one. Wind very relaxed and one being very compressed

Meopta R1 probably 10 to 15 years old id guess
It's not on the end of adjustment I don't think never hit it during adjustment or re zero

 

[webley701]

Powder burn rates will vary with ambient temperature so you will get a difference between winter and summer. Depends on temperatures and powder type but I recall it can be 1 fps per degree Fahrenheit.

In winter air is also a lot denser than in summer time so more air resistance to flight of bullet - one of the reasons most ballistic Apps include temperature and air pressure as variables.

No changes in atmospheric conditions encountered in the U.K. are going to cause a 2” difference in MPI at 100 yards.

 

[shakey jake]

Struggling to tell if your serious or not

I think hes joking no way they can affect point of aim, unless you are a sagitariuos with jupiter rising?

 

[Smellydog]

Depending on moon phase and position my Mrs moods swings can be huge!

 

[shakey jake]

Depending on moon phase and position my Mrs moods swings can be huge!

Thats because you stay out late on a new moon and wake her when you return

 

[Heym SR20]

No changes in atmospheric conditions encountered in the U.K. are going to cause a 2” difference in MPI at 100 yards.

Have a watch of this video from Vihtavouri on this subject.



N100 series powders - a 10°C change in temperature will give a 10 m/s change in velocity of the bullet.

So between summer and winter you could see a change in velocity of 20 m/s. This could substantially change point of impact.

Bear in mind changes in internal pressures and velocities will have an impact on how the rifle recoils which also has a significant effect on where in the recoil cycle a bullet leaves the barrel and thus point of impact.

Then you add in the effect of temperature of air pressure. Cold air is significantly denser than warm air. Cold air will thus impart significantly more resistance on a bullet, thus it will loose velocity more quickly and thus hit a bit lower. Start it off 20 m/s slower and effect is further enhanced.

 

[hendrix's rifle]

Have a watch of this video from Vihtavouri on this subject.



N100 series powders - a 10°C change in temperature will give a 10 m/s change in velocity of the bullet.

So between summer and winter you could see a change in velocity of 20 m/s. This could substantially change point of impact.

Bear in mind changes in internal pressures and velocities will have an impact on how the rifle recoils which also has a significant effect on where in the recoil cycle a bullet leaves the barrel and thus point of impact.

Then you add in the effect of temperature of air pressure. Cold air is significantly denser than warm air. Cold air will thus impart significantly more resistance on a bullet, thus it will loose velocity more quickly and thus hit a bit lower. Start it off 20 m/s slower and effect is further enhanced.

Watched that one a couple of days ago. I was very surprised to hear them say 500 series is more temp stable than 100 series. From my testing, that's not what I've found but they're the experts

 

[webley701]

Have a watch of this video from Vihtavouri on this subject.



N100 series powders - a 10°C change in temperature will give a 10 m/s change in velocity of the bullet.

So between summer and winter you could see a change in velocity of 20 m/s. This could substantially change point of impact.

Bear in mind changes in internal pressures and velocities will have an impact on how the rifle recoils which also has a significant effect on where in the recoil cycle a bullet leaves the barrel and thus point of impact.

Then you add in the effect of temperature of air pressure. Cold air is significantly denser than warm air. Cold air will thus impart significantly more resistance on a bullet, thus it will loose velocity more quickly and thus hit a bit lower. Start it off 20 m/s slower and effect is further enhanced.

You are clutching at straws here as regards trying to find atmospheric effects (including temperature) that will make 2” difference in mean POI at 100 yards in the British Isles (which is where the OP lives).

Furthermore during many years of shooting NRA TR & MR, (where the sight setting & point of impact for every shot is recorded by the shooter) - in conditions ranging from blistering hot to below 0 C, the adjustment in sight setting required between hot & cold even at mid to long range is still relatively minor. The correction increases disproportionately as distance increases but at 100 yards it’s not even a 1/4 minute click of the sights let alone 2’’.

The OP should look elsewhere to explain the shift.

 

[Heym SR20]

You are clutching at straws here as regards trying to find atmospheric effects (including temperature) that will make 2” difference in mean POI at 100 yards in the British Isles (which is where the OP lives).

Furthermore during many years of shooting NRA TR & MR, (where the sight setting & point of impact for every shot is recorded by the shooter) - in conditions ranging from blistering hot to below 0 C, the adjustment in sight setting required between hot & cold even at mid to long range is still relatively minor. The correction increases disproportionately as distance increases but at 100 yards it’s not even a 1/4 minute click of the sights let alone 2’’.

The OP should look elsewhere to explain the shift.

Oh bully for you. The OP did say that it is a seasonal thing that happens as it warms up and gets cold.