Hot air vrs Cold air bullet drop
Reposting here ... for the experts . .
I am trying to understand the performance change across 50 degree temp change.
Two months ago I zero'd my 308 at 100 yards. It was near 100 degrees here. 3 shot group was 0.383 MOA.
This morning it was 48 degrees. FIFTY degrees cooler.
100 yard result was nearly 2" HIGH. SAME AMMO. Same everything.
200 yards was near 2" LOW from the 100 degree results.
Tikka 308, 1/11 twist, 22.4" bbl
Sand bags fore and aft.
Vortex FFP 4x16x44
Remington Corelokt TIPPED Boat tail - 150 gr
Anyone have an explanation for me?
Thank you!
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If you had a chronograph, it could show if the temp is causing variations in your velocity. Here is an article explaining what "might" be happening;
An Official Journal Of The NRA | Can Outside Temperature Affect Cartridge Velocity?
If you zero your rifle in the fall, then shoot prarie dogs in the summer, will your rifle shoot to the same point-of-impact?Your 2" high at a hundred could be because with a slower round the barrel rises a bit more in recoil before the bullet exits. Lower at 2 hundred is the sign of less velocity. Sounds like you are still minute of deer so my advice is to not worry about it much and go kill a big one. For peace of mind you could sight it in for the temps you expect during hunting season. Bob
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An interesting observation. The SPEER Reloading Manual states that air density affects the drag on the bullet. Dry air generates higher dran than humid air slightly affecting the ballistic coefficient. It gets complicated. Good luck.
If you have a wood stock the change in tempeature and humidity could change the pressure on the barrel IF it is bedded tight against the forend wood.
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There may be some difference but certainly not enough to change zero by 2" at 100 yards. Two inches higher @ 100 yards and 2" lower @ 200 yards. That's a HUGE difference in trajectory on top of the change in POI.
ADDED: If your powder is that temp sensitive, I'd change powder.
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The bull shooter doc made sense. The rated FPS of 2840 most likely got a lot faster when it was 100 degrees out - sun hitting the cartridges heating them up before getting into the rifle, then a hot BBL after 15-20 rounds. And then a lot slower 50 degree cooler - could have a swing a close to 200 FPS. It identified a 2" difference - but that was at 300 yards.
In conclusion, there WILL be POI changes with 50 degree change in temp. Best to zero for each AT the location/temp.
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Air temperature can make a difference, but 2-inches at 100 yards for a 50 degree change is a lot. I'd be looking for what was different with my gun, ammo, wind, my shooting technique, etc.
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If the powder you're using is that temp sensitive and you're close to a max load, just be thankful that you didn't load them when it was cold out.
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These are FACTORY rounds.
Remington Core Lokt TIPPED 150 gr.
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I pulled up the Winchester Ballistics Calculator which allows you to change the temperature and get a handle on the change in trajectory. Based on that information the difference you experienced is not due to temp change. The top was the expected performance at 104 degrees and the bottom is at 46 degrees. You can see that there is a change in the downrange velocity and energy predicted but apparently not a change in the POI. I did one additional simulation at -21 degrees and it did alter the POI at 200 yards by .2 inches. So the simulation does recognize the temperature influence.
The change in velocity at 100 yards is just .8% (less than 1%) in the cooler more dense air and just 1.5% slower at 200 yards.
https://ballisticscalculator.winchester.com/#!/select
Michael
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