I note when shooting in the morning the POI change to that of the afternoon, is there a rule of thumb regarding making adjustment to the scope over the course of the day?
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The rule of thumb of POI and temperature/altitude change for airguns?
- Thread starter Goldwing
- Start date
I have my dope card calibrated to 63 degrees ( the year round average here) for every 25 degrees I add or subtract .1 so a 2 mil holdover at 80 yrds becomes 1.9 at 88 degrees and 1.8 at 113 degrees and yes it does vary 100 degrees in the year here
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No. If you know the distance, and if you have a good scope, shoot 2 shots, if they are together, adjust the scope and you're finished. You can have a smart phone full of charts and graphs. You look up your answer and shoot. It will be wrong, so then you make the adjustment you could have made 5 minutes ago. Trust me, I've been doing stupid things a long time.
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Elh, that was really funny! Pretty much my experience too. A little head wind or tail wind and elevation gets impacted along with windage. I always figure on having to adjust a click or two for prevailing conditions.
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I tend to agree with this post. I get a lot of temp and humidity changes in my region. And things like setting up in the shade to shoot, will inevitably shift once the sun comes around the trees.
A couple shots in test target is my best bet for adjusting to the day.
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My group of buddies and I experience that same issue when starting to shoot in the cool mornings and as the day wears on and gets hotter, the POI changes (along with sometimes wind conditions).
We figure that it's not just the heat of the day making the air temperature hotter, but the rifle actually warming up. Considering that there are many different metals/parts involved, and all of those metals/parts heat at different rates, it seems that is likely the biggest factor, though not proven yet.
Having a black finish doesn't help that factor as that color collects heat faster than other colors.
The shroud is first to heat up and depending upon the angle of the sun's rays, it could warp on the heated side via expansion, thereby affecting the liner within as it is connected at some point (this with FX rifles).
So, in our case at least the POI changes throughout the day depending on just how warm it gets. Like you, our weather commonly starts at 65° and easily climbs into the high 90° and often +100°. All the while heating up the metal/parts at various rates and causing POI movement.
That's our theory anyway. We have not experienced any real movement due to elevational changes though.
mike
We figure that it's not just the heat of the day making the air temperature hotter, but the rifle actually warming up. Considering that there are many different metals/parts involved, and all of those metals/parts heat at different rates, it seems that is likely the biggest factor, though not proven yet.
Having a black finish doesn't help that factor as that color collects heat faster than other colors.
The shroud is first to heat up and depending upon the angle of the sun's rays, it could warp on the heated side via expansion, thereby affecting the liner within as it is connected at some point (this with FX rifles).
So, in our case at least the POI changes throughout the day depending on just how warm it gets. Like you, our weather commonly starts at 65° and easily climbs into the high 90° and often +100°. All the while heating up the metal/parts at various rates and causing POI movement.
That's our theory anyway. We have not experienced any real movement due to elevational changes though.
mike
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Newbie experience:
Earlier this year when the temperature was ~ 55F I set up my Huma regulated 25 caliber Marauder to 885 fps with JSB Exact King 25.39 grain pellets. 50 yard groups were great with pellets sorted with a pellet gauge and by weight.
Temperatures have now risen to between 80 and 90F, and the rifle was sitting in my detached shop which heats up to nearly ambient temperatures. It was sitting for a while with a full charge, but the pressure gauge was at 2000 psi (regulated) as per usual.
Checked 50 yard zero again and accuracy was not good with same sorted pellets. Set up the chronograph to verify and first three shots were between 939 and 945fps! I didn't expect this much of a change. That extra 60 fps may explain the poor grouping.
Now planning to set it back up to 885 fps at current temperatures and verify zero and grouping.
Earlier this year when the temperature was ~ 55F I set up my Huma regulated 25 caliber Marauder to 885 fps with JSB Exact King 25.39 grain pellets. 50 yard groups were great with pellets sorted with a pellet gauge and by weight.
Temperatures have now risen to between 80 and 90F, and the rifle was sitting in my detached shop which heats up to nearly ambient temperatures. It was sitting for a while with a full charge, but the pressure gauge was at 2000 psi (regulated) as per usual.
Checked 50 yard zero again and accuracy was not good with same sorted pellets. Set up the chronograph to verify and first three shots were between 939 and 945fps! I didn't expect this much of a change. That extra 60 fps may explain the poor grouping.
Now planning to set it back up to 885 fps at current temperatures and verify zero and grouping.
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In years of competitive FT shooting never have found or used any charting or algorithms to cheat these changes ....
Actual shooting at that day, those conditions, the elevation etc tells the tale and you adjust accordingly.
Actual shooting at that day, those conditions, the elevation etc tells the tale and you adjust accordingly.
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I do not shoot FT but rather hoped to set the gun up for hunting over the summer and minimize the changes needed week to week or month to month.
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