An extremely realistic box mag in terms of external appearance. Powered by a 9v battery, the mag also employs metal contacts at the end of the feeding mechanism that are used to signal the box mag the gun is firing, and it needs to wind up. An extremely creative design that eliminates the need for a winding button or unrealiable sound-based winding mechanisms employed by the Classic Army and A&K SAW box mags. The mag does have a back-up manual winding button as well.
Loading this thing isn't easy. A small swing-out door is accessed by sliding off the plastic side plate. I've found that pre-loaded old prescription medication bottles is the easiest way to pour bbs into the mag for field reloading.
As great as the theory is, in practice these mags are far less reliable than either an A&K or Classic Army SAW box mag. The winding mechanism is very prone to jamming and I've never cycled more than 2000 rounds without a jam. Occasionally they are so serious the entire mechanism needs to be taken apart to fix the problem, making field repair impossible. These problems are so severe that even though I carry a backup mag, I have had both go down on the same day and not be repairable on-site.
Basically, the lack of reliability of these box mags is the one and only drawback of the G&P Stoner 63. If G&P can get these problems resolved the Stoner will be far and away the most effective airsoft platform on the market.
Elimination of need for winding button.
Ergonomics (the Stoner is much easier to carry with this box mag than an M60 or SAW)
Difficult to reload