Why the change of temperature affects accuracy.

[skorec]

I have noticed more times that if  I  bring the PCP  gun and pellets  from  indoor temperature (22C)  to outside winter temperature ( 0C ) the accuracy failed down    after 20-30 minute.

The group opened  and goes 1 MOA lower.

Why ?

[WeeHeavy]

I'm sure there are many reasons, but what springs to mind is that cold air tends to be denser than warm air, thus the pellet may lose more velocity over distance in cold air than warm air.
Edit: Also, Pressure in the tube is affected by temperature. Cooler temperatures will reduce the pressure somewhat.

[moorepower]

More O2 in the air= more drag.

[AlanMcD]

Everything on the gun will shift a bit as it cools down, and it will take quite a while to stabilize at the lower temperature - 20-30 minutes is not long enough.  The biggest factor is the scope, as the optics will be changing the whole time - there are several elements in a scope and they all change at different rates.  And when it finally stabilizes it will almost certainly result in the pellets having a different POI.

[Ribbonstone]

This made me feel like old age crept up on me..at least for a bit.

Do you wear a coat in the cold?

Strrted with springers, but even PCP's do respond a bit to changes in hold (at least to the small amount you mention....1MOA is like 1/4" shift at 25 yards).

Inside, was in shirt sleaves...outside was in a coat.

So I tried sweating a bit inside with a coat then going outside with the coat on.  Then tried shirt sleaves inside and freezxing a bit in just shirt sleaves outside in the cold.

At least half (if not "all") of the shift in POI was due to the layer of pading (the coat) between me and the butt stock...basically was holding the rifle "looser" (unintentionally) with the thick padded coat.

Anyway, it's something to investigate before you go blaming the rifle.

[rsterne]

It could also be lube on the hammer getting thicker, slowing the strike down and causing a decrease (or inconsistency) in velocity.... Use a Chrony to check for this problem....

Bob

[Ribbonstone]

Yes...warmth does make a difference to lube viscosity and the flexibility of various seals.

Air is not totally insensitive to pressure changes due to temp...it's just quite tolerant of normal changes.  But if you are running closer to the edge, where the speed really is the expected best when up close to the max. pressure, it can certainly show up as a speed reduction.

But we have electric heaters and freezers to that temp. that kind of stuff (depending if your significant other will freak out about an airgun laying on top of the frozen pork loin/12 pond whole rib eye or not).

[skorec]

Everything on the gun will shift a bit as it cools down, and it will take quite a while to stabilize at the lower temperature - 20-30 minutes is not long enough.  The biggest factor is the scope, as the optics will be changing the whole time - there are several elements in a scope and they all change at different rates.  And when it finally stabilizes it will almost certainly result in the pellets having a different POI.

Interesting opinion. 
..........

YES .
At beginning  ( with  warm gun, scope and pellets  )   10 shots was at center  inside 1MOA at 77 yard.
Then I did cca  30min pause. When I return to shooting the group open to 2-2,5 MOA and went 1-1,5 MOA lover.
YES
I will need to taste it more longer time and with   chrony  too.

[skorec]

I tasted it again at  3C  :

After 10-25  minutes the group start to open and  pellets   was dropping down cca 1-1,5 cm however the speed was still between  295-300 m/s.
After 50-60 minutes  group closed back  and pellets dropped up to 2cm of the middle and sped dropped down to 292-297 m/s.

It really  seams that cooling  down  scope entrails effects the POI more then the changes at gun and pellets.

[Ribbonstone]

Now that has me wondering.


If you were to have the rings as far spaced apart as possible (like parade-rest) vs the rings close to the middle (attention), would it be better or worse?

[oldpro]

 When I was testing valves I put the guns action in the fridge for 35 minutes(no lubes on hammer) In every case there was a 50+ FPS drop

[Tweeter]

When I was testing valves I put the guns action in the fridge for 35 minutes(no lubes on hammer) In every case there was a 50+ FPS drop

Then maybe it's as simple as them pellets don't like to be shot at slower speeds?  Maybe they like 900fps but not 850fps type of thing?  Or maybe all of these things combined (scope "shifting", airtube contracting and speed dropping)...?

[moorepower]

I tasted it again at  3C  :

After 10-25  minutes the group start to open and  pellets   was dropping down cca 1-1,5 cm however the speed was still between  295-300 m/s.
After 50-60 minutes  group closed back  and pellets dropped up to 2cm of the middle and sped dropped down to 292-297 m/s.

It really  seams that cooling  down  scope entrails effects the POI more then the changes at gun and pellets.

.                                                                                                                                                 I have hunted at -20 f and have never had a scope change zero

[skorec]

I am not sure if it can depend also with using  magnification I use  mag 32x ( Havke 8-32x56 SF 20xHalf Mildot )  . Which magnification do you use ? 


I am also not sure if  temporary ( between 10 to 50 minute after go out into cold air with PCP gun ) inaccuracy dependence  mainly  by scope . 
Important only is to know that  POI changing of gun lasts cca one hour.

Notice.
In/at  my  case/gun  the pellets   start to go down and right cca 2cm and after 1 hour returned to middle.

[squirrel_hunter]

I tasted it again at  3C  :

After 10-25  minutes the group start to open and  pellets   was dropping down cca 1-1,5 cm however the speed was still between  295-300 m/s.
After 50-60 minutes  group closed back  and pellets dropped up to 2cm of the middle and sped dropped down to 292-297 m/s.

It really  seams that cooling  down  scope entrails effects the POI more then the changes at gun and pellets.

.                                                                                                                                                 I have hunted at -20 f and have never had a scope change zero

I never had a problem with any shift while hunting,the rifle always found the squirrels head just like I planned it no matter how cold.

[moorepower]

I am not sure if it can depend also with using  magnification I use  mag 32x ( Havke 8-32x56 SF 20xHalf Mildot )  . Which magnification do you use ? 


I am also not sure if  temporary ( between 10 to 50 minute after go out into cold air with PCP gun ) inaccuracy dependence  mainly  by scope . 
Important only is to know that  POI changing of gun lasts cca one hour.

Notice.
In/at  my  case/gun  the pellets   start to go down and right cca 2cm and after 1 hour returned to middle.

6X24 on a .204 Ruger at 400yards Not an airgun, but at 400 yards it will show up really fast on a crow if the scope is changing. I am not buying, renting or leasing that it is a scope problem. The colder it gets, the more O2 in the air. Would your gun shoot the same at distance at 4000FT as it does at sea level? The density altitude of where you are shooting can change that much and more from a cold winter day compared to a 100f humind day in the summer. The air is actually thicker on a cold day. The air will be denser in the tube at equal psi from a cold day to a hot day. Although its nowhere as significant comparing O2 to N the same laws of physics apply. More volume will be traveling through the same orifice. I am sure there "might" be mechanical issues also, but air density is where it is at. The power of the denser air will increase energy is greater than the reduction in drag from the thinner environment in which the pellet is traveling.

[skorec]

Today was dead calm and therefore I was testing outside cool effect to the POI at 77 Yds.
I found out that pellets are shooting more down after one our however the speed not decreased at all.

Notice
My standndard seams  to be   deduction of two flyers and one pushed from 10 shots.