http://www.k-var.com/shop/product.php?productid=17026&cat=330&page=1
I don’t get it. 
5/10 round mags are much easier to use on the bench for sighting in. Also most states frown on hunting with a 30 rounder in place.
Also, in CA there is a ten round limitation on new semiauto rifle magazines. These mags only appear occasionally.
It’s because it takes just a little to get our friends in Kali excited considering what they are being subjected to. 