My stock guns, especially after reproof, are very tightly jointed on the bite and although I ease this jointing to make them easy to open without undue force, they still need to be 'shot in'.
Allowing the bolting to snap shut (not 'slam' shut!) means they are bolted to the correct degree by the sprung leverwork and they will settle in correctly.
Damping the action of the spring can only make then bolt at a lesser degree and I would be concerned that a 'step' could be created on the botling surface making it increasingly likely that the gun will not bolt fully.
Snapping a gun shut (repeat, not slaming) will allow the spring to do its job much more consistently than your thumb could ever discern.
Once a gun is fully 'shot in' I doubt whether it makes any real differance. If loosely bolted on the first shot, the recoil will probably settle the bolting to where it should be and the second shot would be bolted where it should be.
If it loosens the bolting, the gun probably needs rejointing anyway!
But then what do I know?
