My case is this:
Limit magazines to 10 rounds, and criminals will do one of two things: 1) use 30-rounders anyway, because there are millions of them out there already, or 2) bring more magazines. I'm sure they don't sit there in their parents' basement thinking "gee, I want to kill 11 people, but I can only put 10 rounds in this magazine." They usually bring bags (or more common lately, load-bearing vests) which can easily hold spares. Which means they are limited only until they reload...which seems like forever in FPS games, but is not much time at all against an unarmed target-rich environment.
Limit magazines to 10 rounds, and most civilians will only load one 10-round magazine. This means that because they are "prepared" for an average break-in, they are not prepared for the situation of multiple determined attackers. Even if you load extra mags, you're probably not likely to have them handy, unless you've prepared a way to get those mags to your defensible position (or have them there already).
So essentially, minor nuissance for the criminal (if followed at all), potentially lethal situation for the law-abiding gun owner.
Now, personally, I don't think you
need a 30-round AR mag, and when I get an AR I'm likely to store it with 20s. I know a lot of people (maybe not a majority but a lot) agree with me on this. However, the fact that I don't need 30, and I don't think you need 30, does not stop me from saying that you should be allowed to have a Beta C-Mag if you want. After all, I think you should be able to have whatever you want as long as you're not in prison and not broke
