If you held the gun upside down, or even sideways, you could hold it in the conventional fashion.
Gruesome to speculate on this, but it does spark my curiosity
Something I noticed in the picture: the index finger is relaxed and straight. Would the fingers of the hand straighten like that in the throes of this sort of death?
Another thing I know from observation; in death, the only things that may prevent a body from completely relaxing are gravity and obstacles. There are no remeining nerve impulses telling muscles to relax. If VF died due to sudden annihilation of the motor control areas in the brain, every part of him would instantly relax to the extent possible.
Rigor mortis would simply, temporarily, "freeze" the body in this relaxed position.