i guess entities of weapons are coded for not to use model sequence from w_ models. but you can include rotating versions of w_ models, you can use them with another entity.
for example make an animated rotating "w_9mmar" model, and name it "9mmar_rotating" and use it with a "cycler" entity. and add a trigger for equip.
http://twhl.info/tutorial.php?id=64i found this, i guess this is about adding a model key that can be picked from the ground.
but sadly i dont have any idea about posibility of deactivating model entity after equipping.