Actually it is not so much the temperature of the barrel as what it does to the plastic sabots, when the temperature of the barrel is high enough to soften the sabot your results will change. I live in AR and some times in the summer shoot in over 100 degrees so I take 3 or 4 muzzle loaders and shoot them one at a time and let the first cool while shooting the others. The only other way to maintain accuracy in that kind of temperature is to artificially cool and that brings on two other sets of problems.