Remix from this talented dude: https://grabcad.com/library/ar15-10-15-20-and-30-round-223-5-56-magazine-stl-s.
Keep in mind, this is yet to be tested at the range. Lots of snow here, so it might be a while. Rifle cycles fine, but I need to see if it holds up.
YOU WILL NEED TO USE A HIGH TENSILE STRENGTH FILAMENT, like HTPLA, or Nylon if you want to stand a chance of getting a usable print. I'm not sure if ABS will even work in this case. You can also cast this in rigid resin, or aluminum.
Disclaimer: if you make a bad choice and screw up your firearm or yourself, I take no responsibility. You make your own choice, and you'll live by it.
Update: Rescaled from standard to metric. Cura will like that better.
Update (1/18/2019): Added 0.65 mm to longer side walls. Follower and floor plate adapter haven't yet been updated to match. You'll have to sand and polish them in, which you should do anyway to avoid binding. Remember to print with plenty of walls to take the sanding.
Update (1/22/2019): I didnt like how much the feed lips flex even with premium HTPLA, so I chose to anneal it. My brand of filament shrinks about 1% on the X and Y axis, so account for that in your slicer if you plan to anneal (I suggest you do if you want more than 3 rounds). Strength was increased and I'm much happier with the feed lips now.
MakerGeeks Raptor HTPLA
I printed the largest part (magazine) upright with a full brim.