Are you saying the round will "cook off" or it simply gets too hot and deteriorates the accuracy?
it could drive the pressure up accelerating velocity which will result in different shot placement vertically
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Or if you are loaded hot already, blow a primer
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One of the tests I've conducted with one of my field rifles is to do exactly this and gauge for vertical shot dispersion such as when shooting during hot summer months
Shoot rifle until barrel is hot, while letting the box of ammo cook in the sun with the lid closed
chamber a round and close the bolt so the round can cook in the hot chamber even further.
then fire that round and check to see how much vertical you just induced
So within that particular test I conducted a worst case scenario of all heat factors contributing to see just how bad it might be.
The particular powder I used was not affected in 95F heat during that test and fired that round right into the group
BUT!
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some powders are more sensitive to this
some are not very sensitive at all
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When you are out in the field, hunting, or shootings dogs etc you don't always have the luxury of controlling the temps of your equipment and ammo.
So you want to test to see for instance, should I lay a towel over my ammo in the sun
should I let my barrel cool a certain amount so the chamber isnt too hot
etc etc
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for instance, lets say you're deer hunting, and you have your ammor on the dash of the truck within easy visual reach
So you see a buck at 300 yds and grab a box of ammo off the dashboard that has been sitting in the sun
might be a bad idea, so you want to test this so you are aware of it.
OR - avoid doing that
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for competition - I'm sure you know how much more critical accuracy and consistency is.