Algodoo

Instructions are also in the save menu.
Create objects near the base of the cannon, and it will launch them (collision should be group A)
Objects to be launched should be smaller than the barrel to prevent collisions with the sides
Only magnetic repulsion is used for both launching the projectile and for keeping the projectile from colliding with the sides of the barrel
Most railguns I've seen on here don't work properly, well this one does, and it's very simple