Tripp Research makes the 52 rounder I was using at Fort Benning. It is just a modified USGI magazine with a milled extension that is quite long and heavy. Double stack all the way down, not a coffin mag. The Surefire magazine is much shorter and lighter than these. Tripp Research Inc. AR-15 Magazines
That quoted MSRP is a bit out there... If they get the 60 rounders down to around $40 each I'd buy a few just for the hell of it. Got any pictures of one taken apart? I'd love to see the guts.