I'm bringing this thread back up because I bought this gun and its giving my the same problem.
I tried adjusting the screw above your trigger finger and switching to a non-beveled insert did nothing. And I don't have any larger o-rings to try. But I highly doubt its the o-ring causing the problem because the gun shoots (i.e. the hammer is released) before I even get the pump handle halfway forward again.
Nothing in this world that's worth having comes easy.