You don't need a 30 round magazine. Actually, I prefer 20 round magazines. Early in the design stages of the AR rifle, the 20 round magazine was the standard magazine. The ammo pouches that the Army gave me were set up for the 20 round magazine, but over time the 30 round boxes have become standard.
What is a magazine? It's simply a stamped steel box with a spring and a plastic device to follow the ammo. That's it. A steel box with a spring. Some of them work well, some of them don't. As a matter of fact, when I'm diagnosing a problem with a semi-auto firearm, the first thing I look at is the magazine. Those that are found faulty are crushed in my bench vise and tossed in the trash. Life is too short to bother with defective magazines.
Last year, POST council here in Louisiana changed their qualification course from 40 rounds to 48 rounds. So, to qualify with the AR, I need a 20 round and a 30 round magazine. "But PawPaw", you'll say, "You're a cop and you should be able to get a 30 round magazine if you need one."
That's true, but I'm one of those guys who likes to pop into my local gun shop and pick up a magazine when I need one. If a magazine ban goes into effect, I won't be able to do that because those steel boxes won't be readily available. I'll have to jump through hoops to get one, specially stamped "Military or Law Enforcement", and I don't like jumping through hoops.
I'm also one of those cops who don't believe in exceptions for police. Police should follow the laws that everyone else follows. And, let me say this one more time: In 30 years of police work, I have never had a problem, not the first single problem, with a law-abiding citizen who had a gun. Law abiding citizens are not the problem.
But, it gets back to the You Don't Need argument. The government shouldn't be allowed to tell me what I need.