So I can't help but think: a magazine isn't much more than a metal box with a spring. Come to think of it, that's all a magazine is. I could very easily manufacture a rough but working magazine - holding as much ammo as I wish - in a machine shop. Apart from the spring, EVERYTHING that I'd need to produce a magazine in an hour or so's time is within ready reach of me.
Hey, I've made knives. Making parts for guns would be the next logical step. And there are many with far greater skills who could produce not just the magazines but full-working guns, and possibly mass-produce them at that.
Not to mention that rapid-prototyping - AKA "3D printing" - is already allowing for production of magazines and other gun parts on your desktop. Before very long if you want a gun, you'll be able to download one from the Internet. Literally.
I'm guessing that if government restrictions are placed on firearms and magazines, that there will be a vast underground market for those produced in home shops etc. And every one of them will be unregistered and untraceable.
I'm just sayin', is all...