The Naval Surface Warfare Center(NSWC), Crane Division(in Crane, Indiana) has long been a breeding ground for experimental/prototype weapons for special warfare. The "Special Purpose Rifle" is an M16-based 5.56mm sniper rifle with with a very specific purpose–the quiet elimination of enemy personnel at medium range. DefRev is guessing that the SPR was developed at the request of the Naval Special Warfare Command.
DefRev has just received word from one of its trusted sources that the Special Purpose Rifle is top notch. Apparently, it performs really well for its given purpose. No more specifics on the SPR, at the moment.