Algodoo

A cannon with custom black powder, a cannon ball and a ram rod.
It comes fully loaded, but after the first shot you'll have to reload (or just click undo, but that's no fun).

In order to fire, just pull on the rope until the cannon fires.

There are reloading instructions in the scene.

Fires at about 50-60 m/s, and is pretty accurate. If you release it from the background, it slides REALLY far backwards, even though the cannon is really heavy (around 700 kilograms I believe), so I guess it'd kill somebody if you shot them with it. (the cannon ball weighs 50 Kg, so it'd better kill somebody when you shoot them)

Updates(s):

V.1.1.:
Added Grape Shot and Bolo (grape shot doesn't work so well)
Improved the rope on the cannon

V.1.2.:
Made the bore much smoother, so now you can fire square projectiles
Added a Packaged Grape Shot that automatically opens up, but only when it hits a certain speed, so it won't open up when you're trying to load it
Grouped the Black Powder together for faster copying
Loaded the cannon with a standard cannon ball, because as s_noonan pointed out, it wasn't loaded (forgot to reload it last update. Oops)