It can be argued multiple ways. Yes, it absolutely impacts the ability to do harm, but not nearly as much as you'd think. I heard a news outlet report that when a shooter changes magazines they call it the "critical pause", and they say it can last between 10-15 seconds. They also say that this is the time when the shooter can be most easily stopped and so that reload time is vital. I've been shooting for over 40 years and I don't think I've ever seen it take anyone 10 seconds to load a new magazine. More like 2-3 seconds, and that's not even trying to be fast. So while I'm not so hard headed as to think capacity doesn't matter, I'm also smart enough to realize that as fast as reloads are, it wouldn't make that much of a difference. Especially if the shooter practices. I know guys that can reload a mag in the blink of an eye. So whether it held 10 or 30 makes little difference. That's one argument. The other is still the same as drugs: you can outlaw them all you want and criminals will still have them. So then you limit the good Samaritan to 10 rounds while the criminal will still have the high capacity. Except the good Samaritan who carries for self defense probably doesn't have multiple mags.