There's no doubt that a cooler barrel will shoot better. Not only does cooler weather promote barrel cooling, but so do thicker barrels. They don't rob as much heat from the exploding powder that should instead be harnessed to launch the projectile.
Any barrel that's not a heavyweight that is being used for firing heavy powder charges needs maximum time for cooling between shots for best accuracy.
Black powder guns develop a lot of heat. When the barrel heats up it's robbing the gun's accuracy. It's as simple as that.
That's one of the reasons why the most accurate chunk gun BP rifles have such thick barrels.
They don't heat up as rapidly and will stay cooler for a longer period of time, especially in proportion to the size of the powder charges.
More powder produces more barrel heat which means more time needs to be allowed for cooling between shots.
That could have also been a factor back in January.
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