just go with, if hi cap mags are only for killing more people then why do cops have them?
if they reply because the bad guys have them too, then they've just conceded that the law has no effect on criminals.
Perfect and simple answer.
After a nauseating debate with my ultra liberal relatives in California recently regarding 2A, welfare, etc., I finally got them to concede with a similar statement to yours, here is what I said in a nutshell;
Drugs are illegal right?, well yes.....then why does the Bay Area have such rampant drug use and drug related crime ?AND
firearms are basically illegal in Detroit right?, well yes......then why is it the murder capital of the U.S.?
These statements finally got them to stop, ponder, and had no response....maybe even saw a little, tiny lightbulb go off in their heads.
Laws are only as good as the citizens that abide by them and evil cannot be regulated by law.