Use four of the winches (7u, 8i, 9o, 0p) to reload ammo and the two lower center ones (nm) to reset the catapult into the armed position and unwind the winches to ensure the catapult arm can launch (alternatively, you can use the momentum of the catapult arm to reset itself into the grabbers by timing the tapping of the launch button which is L, allowing the two winches to be constantly unwound or removed to reduce the steps to launch)... There are six winches, left to right respectively the controls are (wind unwind, wind unwind etc...) 7u, 8i, nm, nm, 9o, 0p. Release the rocks with grabber controls Keypad 0 and Keypad . for left and right respectively, make sure to release them again upon next reload if it catches onto the machine with no rock in the grabber. I like to grab two rocks after the initial loading of the machine and as I go so that the grabbers have something in them and will not grab the machine. More detailed tutorial on reloading below.
Forward: Up arrow
Back: Down arrow
Left: Left arrow
Right: Right arrow
Raise front pivot: R
Raise back pivot: T
Lower front pivot: F
Lower back pivot: G
Winches: As above (7u, 8i, nm, nm, 9o, 0p)
Release grabbers on winches: As above (Keypad 0 and Keypad . for left and right respectively)
Launch tension spring: L
Launch release: V
RELOADING involves first dropping the winches into the rocks and picking up one in each winch. Then wind up the winches, and when the rocks are raised, wind the other winches and unwind the first winches to transport the rocks to the high winches. Once at the high winches the rocks can be lowered into the holders on the catapult arm. Once in position just drop the rocks into the holders using the grabber controls on the winches.
Very fun to build and learn from. Slow to reload, but if you're into this kind of stuff it can be fun to do.
This took a few hours from scratch. I am new to Besiege I got the game last night and finished it using basic machines of my own creation, having only seen a few videos of the game to go by. Built in an un-modded version of the game.
Enjoy my build and thank you for visiting my post.
P.S. You can run over things in it too.