erm, i don't know if you can
someone correct me if im wrong
but i think what's there must stay there for animation as well? unless you can script it out, but i dont know if you can
usually you have the gun as a separate model and use a script to attach it to the character
if you can't be stuffed separating the gun now you could shrink and hide the gun in the body for that frame? not the best fix, but if it works in this case it might save you time.
that's unless you want the body to then fade away? then you'd see it inside also fading away