Algodoo

A.K.A. Fat Man. It is a gun from Fallout 3. It doesn't have very good range so you're welcome to improve it. You can also try to make the explosion a mushroom cloud, because I couldn't really think of a way to do it. The instructions are with the scene. Here's a tip: After launching it, press undo twice (or until you undo the Sim Start) or it will tend to fail.