Algodoo

Not to scale, as far as I can tell the rolling wheels don't scale down properly because of poor collision detection at smaller sizes.
NASAW = North American Sliding Axle Whipper, mechanism created by Urban Seige.
600kg CW, 4KG projectile for 150:1 mass ratio. About 50%-68% efficient, depending on simulation frequency (Rollers don't fall through track at 600hz+)

To fire, erase the big fixate on the throwing arm. Release angle can be adjusted by moving the big kill block.