The issue with the MAG mags fitting / feeding properly into a KWA is well documented on the KWA AEG tech. forums.
Basically, KWA can't guarantee 3rd party magazines fitting properly because some companies will use a casting mold for their parts after the lifespan of the mold (read: the mold is good for 5000 units, but some manufacturers will save money by producing another 1 or 2 thousand more units before making another mold). So there's no way to tell if you bought some of the 5000 before/after the lifespan of the mold is out of spec.
In my experience, MAG midcaps are good for the money, and they will fit fine. If they don't feed, it's because the feeding tab on the MAG mag is just *slightly* shorter in length than the KWA spec. I fixed this by shimming the mag-catch notch on the magazine itself, with some clear tape, which causes the magazine to rest higher in the mag well when seated properly. This promotes the hop up chamber to press into the feed tab more, giving better feed.
