prolly due to different lenghts. look at the heavy AT launcher of BWmod/mmp basic pack: the weapon flips forward when fired, cos it's shorter then. ofp somehow levels weapons apparently.
so my suggestion is: make a model that includes both states, loaded and fired, then save them
twice. for the "loaded" model texture all parts belonging to the "fired" weapon with a transparent texure. vice versa for the "fired" model.
selecting vertices in o2 and hitting "hidden vertex" in the vertex properies window might do the job for other models than weapons too, but as this model is a weapon, i'm not sure about it cos ofp apparantly handles weapons different than other models. (
edit: just tried it: hiddenvertices work even for weapons )
anyway... on the level of vertices and faces both models will be identic, so they won't flip back and forth when interchanged. - but on the "outer appearance" level one is loaded and one is fired.
i haven't tried that yet, but i'm pretty sure it works