I'll have to agree and disagree with Mac at the same time. Sounds like you should just get a gun doc to do it. the $30 you spend will save you hours of frustration, and you'll have a working gun. A good way to learn how to work on a gearbox is to watch someone else do it properly.
Your descriptions of the problem are far too vague for anyone to diagnose through this thread.
Guardians of Asgaard - KF32
VFC H&K 416C
VFC KAC SR16 JL Billet SBR
TM FN Five-seveN