Simple—he stole it from an enemy as a souvenir. Besides, it only makes sense that he'd read up on the opposition in his spare time.
He could be preparing to escape in case one of those ways are trying to be applied.
This might explain how he knows the recipe for "Back Bay Bayou Bunny Bordelaise a la Antoine, and I don't mean Antoine of Flatbush!
Why is it that in "Baby Buggy Bunny" they edit out the scene where he shakes Finster, but not the scene where he puts finster in a washing machine and then throws him at the ceiling and "forgets" to catch him? Keep in mind, Bugs has figured out that Finster is not really a baby at this point. Even if it's because it looks like child abuse, so is putting the kid in a washing machine!
Because shaking him isn't outrageous and more of an issue since it could be seen as normal or imitated. Putting him in a washing machine is obviously not good, and less likely to be imitated or misconstrued.