"I purchased five of these over a year ago and have used them at skirmishes or ops on an almost weekly basis. My son and I have used these with various M4s (Ares, TM, JG, G&G), DE 416, G&G FN2000 and the PTS Masada without problem. The outer shell is high quality metal and have yet to see one crack or break. On the internals, I have not yet been forced to take one apart to fix an internal problem or jam. The only issue - and I don't blame the magazines - it that I jammed one into a KWA M4 and it apparently pinched the mag, as it now does not seat tightly in my Ares M4. For the person who commented on the JG - these were a tight fit, but they fit. Before you spend much, much more on the 600 round Surefire "fat mag" replicas; consider buying a couple of these 450 to 500 round mags. "