This is being written in late July, and even here in the Missoula Valley of western Montana it gets hot this time of year. Today, the high will be in the mid 90s.
When it is this hot, after just a few shots the barrel of a muzzleloader will heat up and holds that heat - especially if you're shooting from an uncovered position. And once a barrel gets heated, accuracy with plastic saboted bullets goes right out the window. A barrel that is exceptionally hot to human touch will make those sabots soft and less resilient. When loaded ahead of hefty hunting charges, a sabot that has been in the barrel for as little as a minute, or less, can become so soft that it simply cannot stand up to the pressures created by the burning powder charge - and it's not uncommon for the sabot to exit the muzzle in bits and pieces. Needless to say, the bullet it held at the moment of ignition is not going to shoot with any degree of accuracy.
One solution to this problem at this time of year is to only shoot during the first hour or two of daylight early in the morning. This is the coolest time of the day, and where humidity is high, temperatures can already be in the upper 70s-lower 80s before the sun even pokes up over the horizon. Maintaining accuracy even for such a short shooting period will still mean giving the rifle at least 10 minutes of cool down time (in the shade) between shots. But to load and shoot any faster is simply a waste of powder and bullets...not to mention your time.
Let's get a discussion going on this topic and see what others have to say. I have a lot more to share, and I'm interested in learning from some of you.