Learn vector math.
You are multiplying something that is in 200-500 units range, by some number. If that number is 5 or so you send arms 2500 units away.
Second are you sure they move forward? Z is UP, x and Y are right and forward, but if you rotated anything in hierarchy those vectors can point anywhere.
Also do debug for your vectors: print valuesm use line trace (with one frame duration) to see vectors in 3d, and ESC+eject lets you see scene from outside from pawn camera.