For when a mouse gun is overkill:
Real Guns has the complete skinny on this.
Interestingly, they also have a calculator that suggests a rifling rate based on length and diameter of your bullet. For grins, I ran my 2-1/2 bore guns through it and am informed that 1 turn in about 150-180 inches will do the trick. That works out to about 45 degrees of twist over the 23 inches of the barrel.