This is entirely too in-depth to be discussing here, because it's all about interpretation of the law. Go to Calguns.net to read up on it if you wish.

In short: The safe route is to break it open and load it from the top.

The "dangerous route" which has yet to be tested in court is: The law defines what steps one must take to detach a magazine to have it considered "fixed." The law does not define how you must re-attach it.

What does this mean? One could argue that by having to use a tool (allen wrench, or even a .223 round) to remove the magazine, you met the criteria of a "fixed magazine." And since the law doesn't define a "detachable" magazine as a magazine which can be INSERTED without the use of a tool, you could, in theory, detach the magazine with the tool, load a magazine, and replace it in the firearm.

The safe route, in California, for now, is to break it open from the top and load it... takes about 30 seconds or so.

FYI: The law states "detachable" means: "Any firearm capable of accepting an ammunition feeding device, not including stripper clips, without the use of a tool or disassembly of the action."