A throat or arterial slash will still allow the opponent enough time to shoot you, unless you were to use your weak hand to force his pistol against his chest just prior to slashing the neck. Think more in terms of where the arms are weaker.    Try to crush the guy's gun up against his chest, and it's going to be a LOT harder to accomplish than pointing it up (the arm is weak trying to pull down, typically).    Failing that, get it to go across his body (the arm is harder to pull back out than in).    Try to pin his arm in such a way that your weak hand is going to be able to pin his string hand, leaving you free to end the threat in whichever way you see fit.