Do not use Excel brand bb, they are smaller than normal bb's and are intended for conventional AEG type loading, KSC/KWA M11 mag works flawlessly with KSC's bb and a spare mag is a must because the M11 is a hose.
First of all this mag is drum tight. No leaks and fills without losing any gas. However out of the box it will not hold bbs. As soon as I release the mag feed spring all the bbs come flying out of the top. I am using Excel .2 gram bbs.