This is the Alangator TriMag magazine holder.  This ingenious device uses three of your Ruger 10-22 factory 10-shot magazines and holds them together in triangular fashion.  It does so by utilizing the factory depressions and shapes of your 10-22 magazine: no modifications or adhesives are necessary.  It assembles in seconds to hold your magazines tight and secure.  Fit and finish looks so good it appears to be a factory Ruger part.  (In fact, Ruger even offers these in their catalog now)  The TriMag is made from the same composite material used in the actual Ruger magazine.  The center bolt is aircraft grade aluminum.  Now you can hold three 10 shot magazines in your rifle at once, and remain legal in all states under magazine capacity laws.  Changing from one magazine to the next is quick and easy, just eject, rotate, and re-insert.  Shooters in competitive matches have had great success with the TriMag, when seconds count and a non-feeding, unreliable aftermarket magazine is simply not acceptable.  I first began playing with 10-22’s over 20 years ago, and went through several high capacity magazines, none of which would feed and function as well as factory.  And now you can carry extra capacity with reliability!  Plink away all day without a single malfunction!