You should probably check out TorkMags.
I've finally bought a product that I have been watching for at least a year. The Tork Mag is a durable plastic mag with two springs in it that lets you put more rounds in a smaller package. They come in 50, 40, and 35 round capacities. This is a well made mag that is probably more reliable than a normal 30 round, not some gimmicky range toy. These are really well made mags that are theoretically immune to binding, and the pricing is remarkably reasonable.