Originally Posted by Murph
It appears to be an older one, but the trigger frame's aftermarket (as you can tell by the PTP logo on it) I wasn't aware they made frames for the sterling. It's probably not firing because it can't cock. I don't see a screw ahead of the trigger frame. That lug that the hammer carrier? (Never learned the formal name for that component) runs through from the pump handle isn't screwed down.
after reading your post, i did a little digging... i think that the frame is for an automag, and the lug isn't catching the sear...