I had some issues with these mags and Mad Bull 0.28g BBs. Not sure if it's the diameter but it appears MB BBs can "double stack" in the mag and jam beside each other. This is especially prevalent when filling the mags completely. The fix is to hit the mag against something to rattle the jammed BBs free and not overfill the mag. This didn't happen with other BBs in the past so I think the diameter of MBs is just small enough for this to happen.