Originally Posted by mtbfarms
I'm using retumbo in a high performance .416 muzzleloader. Pushing a 375g cutting edge copper @ 2805 fps at 25f and 2745 fps at 70f. The hotter the slower. Colder faster. I'm getting great accuracy just don't like the velocity change with temp.
OK, as a guess you must be in the 140-150 grain range with that load.
Unless you have powder cavity larger than the bore diameter that powder column will be about 4 3/4 inches long.
Being a "straight case" it is hard to maximize efficiency.
Have you played with different primers?
If you switch to H1000 you will need to reduce the load to keep the pressure about the same and then your velocity will almost certainly be a bit lower too.
Personally if the load is accurate at the temperature extremes I would add the info to my database and just adjust!
Summertime you add whatever clicks are needed to compensate for the 60fps and shoot.
You will have more variation than that when you go up/down a few thousand feet or shoot during a low/high pressure system...IMO.