Well, I just so managed to find out how to build a cannon without becoming a supporter - without having to shell out wads of cash. How?

Well, make a small square-ish rectangle, and a smaller squarish rectangle. (The smaller one must be able to protrude from the bigger one with the centerpoint on the bigger one.) Make both of density 1. Move the small one to where the centerpoint is on the bigger one, and insert a rotary joint on the centerpoint (of the smaller one) with the following specs:

MOTOR ON
STRENGTH - 30
SPEED - 30
FLOPPY JOINT
AUTO ON CCW
LOWER LIMIT @ -9 degrees.

Copy and paste this on the same centerpoint about 15 times, and then put the larger squarish rectangle on a form of support - even if it is just a fixated line attached to it. Now, make a rectangle - large enough to be able to touch the "firing pin" you just made, but small enough to where the larger square-ish rectangle is bigger than it. Make the length about 4 times its width, and put it on the support to where it will not accidentally touch the firing pin. Go into simulation.

Touch the projectile you made to the firing pin, and see if it launches. If it doesn't, try again. If the firing pin flies off, reduce the density of the projectile and try again.

For more help, look for "how to make a cannon" by the author, cavancai.

~WK1