Apparently it's the only one in civilian possession. I'm told that the military told Barrett they wanted a cheaper alternative to whatever their current anti-air weapon was at the time. Barrett came up with this, made 10, showed the military, who responded with "Nevermind, we'll pass on that".
(Bandwidth beware, lots of big photos)
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=322952358
(Bandwidth beware, lots of big photos)
http://www.gunbroker.com/Auction/ViewItem.aspx?Item=322952358