What about a compromise between the ideas of it deflecting grenades and increasing spread?
This would be a major change to the game, i suppose, though i haven't spent much time thinking this through.
So, you increase spread for a starter, no one has come up with a counter-argument for that yet unless i am mistaken.
Then you could have something along the lines off:
Give grenades a sort of health, you shoot them enough and they blow, this includes m79 grenades.
For most guns this would be a very minor change, except for some sort of super hero who can hit an m79 grenade three or so times before it blows his face off.
But for the shotgun, with the increased spread, it could just be shooting at the right time and hoping for the best, giving the shotgun the ability to counter grenade spam.
Just speculating and talking out of my ass here though, as i haven't really thought this idea through much other than as i write this down.
Thoughts?