I've shot them with just about every muzzleloader I own. Large caliber with reduced charges (30-35 grains of 3f) is perfect for head smacking. I smack heads with the small cals too, both 32 and 36. In those 20 grains of 3f is starting to feel like too much, because if the shot wanders down into the neck it not only just about takes the head off, it also bloodshoots the whole front shoulders.
In smoothies I've come to rely on nothing smaller than #6, and I like #5 better. Howcum? Because anything smaller than #6 leaves a lot of pellets behind for your dentist to worry about, and I prefer #5 for a little less pattern density so fewer pellets land home, but even one almost anywhere in the critter will do a good job of anchoring a bunny.