ok first are you still useing the xmodel tool by dielogic?, if so scrap it ! no offence to the guy who made it ,it was great to have when noone else would step up and do something better however that time has come and so i point you here
http://www.itsmods.com/forum/Thread-Prev...Utils.html
please download this and begin again
you will find it much better and it works for everythig , now after you have done that you will need to bind the rig to the defmesh ie the gun there are tuts for this on youtube ill try to find them for you .
or if your just moveing guns from one game to another you can just use the bind.mel script made by toms xmodel tool.
edit here
http://ugx-mods.com/videos.html
watch the custom weapons in maya videos they should help you
if after all this you still fail you can send me the weapon / textures and i will use them to make a tut on getting a weapon ingame from scratch rigging,binding,exporting,importing,setup a rig for anims,make a simple anim,export,import and wins, all the things needed to have a truly custom weapon ingame , ether ways hope it works out for you