I had an issue with King Arms mags in my Real Sword Type 56. The stock RS mag fits without issue. It wobbles, but feeds fine. The KA mags (plastic) fit tight, and the tab was too fat to actually fit under the mag catch on the gun. I had to dremel the tab thickness down in order to make it fit (see the mag section of my Type 56 review -
http://www.airsoftcanada.com/showthread.php?t=52250)
Perhaps this will help, as it sounds like a similar problem to what you're encountering.