"Arash"Hi
I know what that.
It work's as a spring there to keep regulator in it's place. It's like a disk.
If you closely look at it you will see it's carve. In this pic I can see it's in a wrong position, turn it 180 degree (head down) and under pressure it will stay right in middle.
With out it, when you shoot the gun regulator goes forward and backward.
"Arash"I suggest 920 ft/s (280m/s). But for achieving this speed first remove the cylinder and loss valve spring for one turn (360 degree), then adjust you hammer spring tension.