It takes more than one clip of Magnum ammo to kill a Charger unless you can nail him in the face, which is incredibly difficult to do if he's charging someone or pounding them into the ground. This is opposed to just about ANY of the Primary weapons, which will drop him in a clip or less, headshot or not.I find emptying the magnum into a special tends to stop them. I'm a reload freak, meaning at any lull I'll be reloading my weapons, so I usually have fairly full mags. The pump shotty is a waste, but I'm trying not to give up on the auto yet.
In other words... don't waste Magnum shots on SI. Pull out your primary... or you WILL get someone killed.