Inert (no propellant and the casing is not primed) experimental Flechette cartridge for the Colt 5.56 Advanced Combat Rifle. Therse were also used in an experimental thousand round shell. Why they were not adopted I cannot tell you, perhaps a bit gimmicky and/or no advantage over standard ballistic tips. These have sold on Proxibid for thirty dollars or more each, I picked up a few as curiosities when they first showed up around here so I will let this go for a little less than that for home repair cash thank you.