Just wanted to repost this excellent gif posted earlier by Steve:

I've never noticed the upward double thrusting movement of JBC's shoulder before but it is clear as day.
I believe the 'doubling' of this movement reveals two very different reactions.
The first reaction is the instantaneous physical reaction when two objects collide. There is
no delay in this reaction, it is instantaneous. This happens when the bullet physically collides with, and destroys, a relatively large section of JBC's rib. This collision causes JBC's torso to rotate, his right side being thrust forward and downwards causing his left shoulder to be thrust backwards and upwards.
The second reaction is his body's reflex reaction to such a massive trauma. This reaction takes place within a z-frame or two after the initial trauma and is totally reflexive, that is to say there is no conscious decision being made, it is an automatic response of the body to trauma. It is at the same time as this second upward thrust of his shoulder that we see JBC's Stetson suddenly leaping up in front of his face.
We see the white of JBC's shirt cuff instantly disappearing below the door frame as the wrist is struck by the bullet.
JBC's body rotates and we see the first 'shoulder lift'.
The right side of JBC's jacket bulges forward as debris is blown out of his chest as his body continues to rotate.
We see the second shoulder lift as JBC's body reacts to the trauma and the first sign of his Stetson as it moves incredibly quickly upwards.
When the Z-film is rolled on from this point we see that JBC's wrist is being held at a really unnatural angle due to the massive damage it has suffered.
Much has been made of JBC continuing to hold onto his Stetson but I believe his hand has suffered nerve damage and he couldn't let go of it if he wanted to.
One last note - using this analysis it is possible to pinpoint the exact moment both men are shot through, between z222 and z223.
There is truly overwhelming evidence that this is when the first shot happens.