Several things: 1. On the outside of the M3 there is a tab on the rear face of the magazine which prevents it from being over-inserted into a magazine well. 2. Since the M3 has that over insertion tab it can fit and function reliably in many non AR15 firearms. 3. The M3 floorplate is more slender and less bulky (makes for easier storage). 4. The M3 has a dot matrix at the lower part of the side of the magazine. These dots are to be filled in to communicate whatever the user wishes. An example would be filling in "300" or "556". 5. The M3 is shipped with a dust cover which prevents debris from entering the inside o the magazine and it also protects the feed lips.