It was called 'The Warlock' and fired a .510" BMG bullet from a necked down 20mm Vulcan round and had a velocity of 3900fps or so. The army originally wanted to test the effects of a hit from 'regular' BMG round fired from two planes going head to head (thus the 1000fps increase needed). The barrels would show EXTREMELY heavy erosion after 100 or so rounds.
Other people have made them also. If I remember correctly, a guy named Skip Talbot (who holds the current .50BMG world records in both light 4" & heavy gun 2.6" at 1000yds with 5 shots) made one, but since the barrel would wear out so quickly, it couldn't hold a competative edge for even one match. They were built on McMillan's 'Fat Mac' action which is no longer in production.
Here is the info I have on it. I only read a quick blurb on it over a year ago and was going from memory. They don't show the weight, but I'm guessing 100# or so.
