That is an intriguing idea, to shoot a bunny with a .50 muzzleloader, and 30 grains of powder.
Thirty grains is what you load a cap and ball pistol with, that is with a .45 ball.
So, 30 grains would be plenty of power with the bigger ball in a rifle.
It certainly would do the job on a head shot.
The ball would be moving so slowly, you might even get away with a lung shot, you would just have to experiment and see.
First you would have to see if you had any accuracy with that load.
You might try 25 grains, or even 20. I don't know, the smallest load I have shot in a rifle is 50 grains, I get good accuracy with that.
You certainly could load it up with some number 6 shot and kill the bunny.
The problem with that is, the shot grips the rifling, and it sprays out in a circle.
That is, all the shot is on the perimeter of the circle, and nothing in the middle.
Shot would only work at close range, there again, you would need to experiment before you went out into the field.