As I have found through sheer sweat and tears, it is always better to pose the figure in ZERO position and fit the gun.  Unless you are using a preset pose for the gun to hand, you should do this.  You set Miki in zero position by clicking pointing at MIKI's Body Part, then go to menus and choose Window | Joint Editor, then click on the zeroing button on the pop up.  Then pose the gun in Miki's hand, lock hand parts, and parent the gun to Miki's hand.  Test out a pose on miki's body, and you should get her hand in a fixed posed, with the gun in it.

Eternal Hobbyist