Well, actually it's about Mach 4.75 at sea level, but it's the fastest thing I've ever seen. (That's with the bronze bullets)
Mach 3.56 (3,975 ft/s) with 270 grain bullets
http://www.statearms.com/mach5.htm
Mach 3.56 (3,975 ft/s) with 270 grain bullets
http://www.statearms.com/mach5.htm