If you intend to kill the cat, you'd be better off using a .22. Contrary to popular belief, pellet guns often don't just "sting". They can cause serious injury or death. In fact, they are often advertised as for killing squirrels and other small game. I actually use a pellet gun myself to kill a chicken for butchering. Mind you, I am shooting point blank to the head, but one pellet is all it takes. My point being here is with a pellet gun, you may end up not killing the cat but seriously injuring it and causing it a slow painful death. Use a real gun or have a talk with your neighbor about your intentions if they don't control their cat.