Wait, you say that won't help? You think that's ridiculous? You say that people who are going to go on a shooting rampage won't follow a law like that?
Well, genius, what the everlivin' fuck makes you think that they're going to follow any other law you pass concerning mag capacity, type of firearm, or background checks?
I saw an article the other day from an AR15 owner saying that "oh, if we just limited mags to ten rounds, it would fix everything because they'd have to reload and it would maybe give people time to charge him and disarm him." And I realize that this has been proposed and shot down more times than I can count, but, uh, in this specific case? The Orlando shooter was texting his wife and posting to Facebook during his rampage. I am pretty damned sure that having to reload a few extra times would not have fazed him. This guy's naivete was super cute but ultimately dangerous and not helpful.
This is assuming that if you suddenly pass a law saying "ten-round mags only!" that the high-cap ones people already own will just suddenly disappear like fucking magic or something. Basically, you're going to hamstring the law-abiding while the criminals and terrorists laugh up their sleeves at your stupid, stupid, knee-jerk Do-Something-Itis.