@JohnO--Yeah, there is one local GM in particular whose hand I avoid shaking. Fist bump! But, I think we can agree that there's much more to this than massive hand strength.
@wtturn--I'm not sure about that. Increasing grip force can add some trigger control problems, as well as solve them. We didn't have time to drill down into his technique, but my next step would be to have him relax his strong hand a bit and work on pulling the trigger straight back without moving the sights. His vertical stringing was almost certainly due to sympathetic squeeze of the fingers of his strong hand.