Buddy, I think what you should use is Blender, I know you might suffer and die from a stroke from the quirky interface and the old Sci-fi menus but it is the best for weapons in my opinion. I rig models all the time and you can too, Nuka-Cola is considered as an aesthetic static object that is just weirdly attached to the gun but that's okay, so when you import (the model you want), slap on the Nuka-Cola you like, then export, and just follow through Nifskope editing then it should be all good. And if you need any help in the G.E.C.K just PM me. Majed.