Saw this on Tactical-Life.com
http://www.tactical-life.com/online/news/magpul-files-for-quad-stack-ar-15-magazine-patent-application-—-click-for-pics/?hp=news_title
Apparently it’s a quad stacked high capacity magazine. I don’t see a total for rounds held but my guess would be twice the capacity of a standard 30 rounder.