Tim, I'm new here as well. Actually joined because of the reasoned and well thought out arguments in this thread. And I want to address the only two things you seem to still be pushing as reasonable restrictions. And I'm going to try to do so by arguing for their ineffectiveness.
1. Banning standard capacity magazines. There are, by some estimates, 50 million+ 30 round AR-15 magazines currently in circulation. Then you have 20 or 15 round magazines. Now throw in every 30 round AK-47 magazine, 12-33 round Glock magazine, plus all the other 10+ round magazines currently in circulation. Easily 250 million+. Probably closer to 500 million, or more. None are serial numbered. None are trackable. Ban sale of magazines over 10 rounds tomorrow, and you know what? It won't matter. Because 100+ years from now, there will still be millions of functioning AR-15 magazines that hold more than 10 rounds. Same for AKs, Glocks, etc.
A magazine is a simple metal or plastic tube with a spring. They aren't hard to maintain or repair. You would have to confiscate them. Besides the violation of the 4th Amendment inherent in that action, they aren't serial numbered or trackable. So even if I have purchased 100 magazines a year for the last 4 years, no one knows. I just have to hide them where confiscators won't find them. So, in short, any magazine ban is a. unlikely to limit the supply by much, and b. virtually unenforceable, since the magazines aren't serialized or registered.
2. Registration comes into play for the requirement of back ground checks for firearms sales between private citizens. What is to stop me from buying a gun for cash, without the background check, after they are mandated? Only if the government knows who is supposed to have what specific guns. By serial number. That means registration. A big central database that can be checked to see if I went through the right channels for the gun, and had it registered to me.
There are 300 million, or more guns in the U.S., in private hands, right now. Who is going to ensure they all get registered? And how, without violating the 4th Amendment? What about the antique guns that were manufactured before serial numbers were required? How will they be registered? And where is the money for this process coming from? None of these questions have answers. The banning of private sales of firearms is impossible without registration, and short of violating several other rights protected by the Constitution, registration is impossible.
I have purposely avoided the "slippery slope" arguments here, or any others, besides practicability of your suggested infringements on the rights of law-abiding citizens. Personally, I don't want any fewer rounds in my magazine than the cops coming to get the bad guy I'm trying to defend against. If with backup and more training, they need 30 rounds, so do I. And I sure as heck don't want the government knowing what guns I have. That's an invitation to abuse, whether by this administration, or one 100 years from now.