Algodoo

The cannon should fire first and then eject the spent shell. You can use another polygon to push it from the front if it doesn't get off easily. This is because the shell would kill you, and because you're wasting muzzle energy.
Can you use 120 or 200 Hz for this? You'll have more simulation accuracy so you'll be able to make the spring stronger, and you'll get more speed

You can change frequency in Options -> Simulation. More Hz means more lag (although you certainly won't notice unless you use a ridiculously high frequency, or lots of objects)