I don't think the base of the grip is considerably more hard and solid then any other part of the gun.
IIRC correctly they tested lethality of a pistol whip in the following episode of Deadliest Warrior
http://www.sidereel.com/Deadliest_Warrior/season-2/episode-3edit looked it up, the episode is correct:
brass knuckles are at: ~30:40
pistol whip is directly after that at: ~32:50
(see how they wield it and what results they are getting in psi (
http://de.wikipedia.org/wiki/Pound-force_per_square_inch))
Yes, they use a revolver, yes it is deadly, no they don't change grip.
Yes I have absolutely no idea if any of you know better.
Kinetic energy delivered by a 'club' is speed times mass².
If you hold it like a hammer your pistol may(!) get "longer" and with a longer 'lever' you get more speed (and less control).
Question is if you can reliably get more kinetic energy if you hold it any different way and if it makes a difference.
I have absolutely no knowledge how one holds a rifle in meelee though.