Well first off, you know that changing the rez files around will only change the ROF and the look of a gun, not the damage or recoil. Just so u know.
Secondly, ur gun is disappearing because the game can't find the model files or can't understand them. I tried replacing the M16A3 with a LAW and the same thing happened. Best rule of thumb for now is us the same type of gun. AR to AR, SMG to SMG etc. this is all assuming that you are doing it right