I might argue that you can't argue with an anti. It depends. Many progun folks were negative in attitude but changed their views.
About the mags - without some god-like power of confiscation, the existing stocks can last for many, many years. The life span is very long.
The rampage shooter, who plans to kill a bunch and die, only needs about two mags for the plan. They will easy to obtain.
IIRC, it took 10 minutes for the cops to arrive at Sandy Hook. Having something like a modern pump shotgun and no opposition at a school, with a backup handgun - banning ARs or AKs is irrelevant.
I took a couple of modern shotgun classes - reloading isn't that slow and being charged by 6 year olds isn't a real threat.