A friend brought over a 999 all apart and couldn't re-assemble the cylinder/frame. There's a small flat spring that retains the cylinder and though it came apart, now won't allow the cylinder to be inserted into the frame.
It's been a while, but I think that spring (called the "cylinder catch") just has to be pushed back into the slot in the cylinder arbor, then the cylinder pushed in over it. Make sure the spring is in correctly.
