In the early days of the Five Seven Forum there were some photos posted of a grenade launcher which had either a mounted or integrated firearm under the grenade launching barrel. Imagine the M-16 with a M203 40mm mounted under it, only reverse their placement and shorten both barrels. I don’t recall what this was, or who was prototyping it, only that it was tan and the firearm took the FN – Five-seveN standard 20 round magazines.
Here is a list of known firearms chambered in 5.7x28mm, both production and prototype. Know of any others?
Excel Arms – MP-5.7
Excel Arms – MR-5.7
Excel Arms – X-5.7R, X-5.7P
FN – Five-seveN, Tactical, IOM, USG
FN – P90, USG, Tri-Rail
FN – PBR [announced only]
FN – PS90, USG, Tri-Rail
Masterpiece Arms – MPA57sst, MPA57sst-ATACS
Masterpiece Arms – MPA5700sst Carbine
Rhineland Arms – AR57 [pre-PWA prototypes]
Savage – Model 25 Walking Varminter
ST Kinetics – CPW [not imported]
VBR-Belgium – CQBW [not imported]
UPDATE 2012 August 2: Masterpiece Arms introduced the carbine version of their MAC type pistol.