This is the beginning of a "rebuild compatibility chart" for a wide variety of magazines, rifle and pistol. Under California law, it's entirely legal to rebuild one magazine into another. See the attached letter from the California DOJ for confirmation.
About Confirmation Levels
Due to my not possessing the majority of the mags in this guide personally, I must rely on hearsay, at least for the time being. To that end, I have come up with a number of confirmation levels which will be used in this guide.
Confirmed: Either I have reproduced the steps personally, or I have access to pictures/video confirming both compatibility and function for all steps of the process. These will be credited.
Documented: I have access to a how-to / writeup for this; links may be provided. These will be credited.
Hearsay: Someone on the internet says it'll work. No pictures, may have personal vouching. These may be credited.
Disputed Hearsay: Someone on the internet says it'll work, another person says it won't. These may be credited.
STANAG Magazine Conversions
Because STANAG magazines are so ubiquitous and the conversions are most popular with these, they merit their own section. If parts will not directly interchange, but another third-party part is available that will work with both magazines, that third-party part may serve as a "bridge part" between the two magazines.
Magpul Ranger Floorplates: These are designed to fit onto 30rd USGI STANAG magazines. Internet hearsay has it that the Magpul Ranger floorplate will also function on the 20rd magazines, thus providing a "bridge" between USGI 20 and 30 round mags.