You can not change this guy's mind..... but remember: he is not the opinion you are trying to change: it's the fence sitters that are watching the two of you argue.
Keep it polite, and rational, based on logic and physics. Let him deal in emotion ....
It depends on the situation I suppose. If the guy has less rounds to shoot before he has to reload, invariably, if the option is there for people to run, more will get out without being shot. Maybe some ballsy guy might overpower him while he's reloading but I never said it was definitely something that would happen.
Ask him this: Would it not be even better if one or more "ballsy guys" would not wait for him to reload, but draw their own guns, and shoot him until he stops killing people? Why handi-cap the good guys by denying them effective tools? What you are calling for is handicapping everyone in the hope that you will handicap the bad guy ..... which is a fool's errand, because the bad guys don't care what the law says.
Even the pacifist Dalai Lama supports the idea of self defense: "If someone has a gun and is trying to kill you, it would be reasonable to shoot back with your own gun."