The letter of the law says the magazine must require a tool to be removed. In spirit, of course, it intended a fixed (welded) magazine, requiring the separation of the receivers to load. You can buy ARs with welded mags, and they are without question legal. However, by the letter of the law, all that it requires is that you need some sort of "tool" to get the magazine out. There is the familiar set screw lock, but there are also ones that use an inset mag removal button, which requires a round or some other sort of tool to actuate, which, by the letter of the law is legal.

When I get around to getting my CA ID, Ima get one with the set screw. That way if the zombies come, I can undo it and kill em all with removable mags h: