By "won't fire" do you mean BBs don't come out or it won't turn over? Just trying to understand the problem.
BBs not coming out in semi-auto might mean a timing problem. What is the fps for a short burst of auto fire? If it is much lower than expected, then that might indicate a timing problem as the tappet would be out of the firing position causing an air leak. Yes, there is timing in a V3 gearbox
If it is not turning over in semi-auto, I have no idea what's wrong there.
Hope this helps more than hurts