Okay thanks.
I took it apart again, and I tried to turn the hop-up. Looks like the previous owner had tightened it too much and it broke. And it feels like when I'm turning the hop-up screw, it's really not there? What allen screw size should I use?
Here's the pics. The pin is still in there.
The rubber piece on the end where the bb is held, I took it off to show the damages.