A mule would probably treat a BB shot like a mosquito. I high powered pellet rifle would certainly be felt more but you might have to get into skin piercing levels if the mule is particularly as stubborn as a .... Oh wait.
I wonder if a heavy duty shock collar and remote for dog training might be more effective, assuming they stay in range. Animals are freaked out a lot more by small, harmless shocks than they are by by regular pain, as they are not accustomed to the feeling. Again, he might just eventually ignore it. Mules are a rugged beast.
Tough question. The mule better hope that the young bull does not have a long memory. He is going to get a lot bigger.
