I think you missunderstood what I wanted to say.

A gun that is fired creates a force directing to the oppsosing direction where the bullet goes to (with the same energy that the bullet got).

The weapon wants to go backwards, but there is the hand which creates a force to avoid this. But since the mass-point of gun and hand is not behind the barrel, but below, somewhere inbetween hand-force and shot-force, both opposion forces create an angular momentum that rotates the weapon.
As your upper arm is not stretchable the gun in your arm has to rotate around your ellbow, and that's the point which is wrong in your animation.

You rotate around the weapon and thus stretch the upper arm downwards, but you have to rotate around the ellbow.


Look at my western shooter video to get what I mean.