I think you may have to deal with the attack first, right?
Cultist attacks, so Harvey can dodge, fight back, or maneuver. Those are all with respect to the attack.
if Harvey grabs for the gun, then he has not dodged, nor has he fought back, nor has he tried a maneuver with respect to the attack. He has ignored the attack and focused on his gun instead. So, I'd let the attack succeed and see if Harvey survives to get to his turn.
If it is Harvey's turn, and if you allow Harvey two actions on his turn (grab and shoot), you can always treat the shot as the second action (i.e., give a penalty die for the shot, and a bonus die for close range. You can then limit the shots he can take during his turn).