I loaded up some .44Magnum cap and ball loads using about 24 grains (weight, not volume) in a .44 magnum case and a .433 round ball with the crimp just over the roundball's maximum diameter. This is a slightly compressed load.
This chronographed at about 1100 fps with one of the lowest velocity variations I have ever seen. They were all within about 10 fps of each other.
I used a Smith & Wesson model 29 with 6.5 inch barrel.
Shotgun primers are not any more powerful than large pistol or rifle primers, it's just that you don't see many muzzleloaders designed to use these primers.