From Fabrique Nationale Herstal (FNH), this ammo is a 28-grain hollow-point, lead free round for use on ranges where lead may be an environmental concern. Boxer primed and reloadable. 

"5.7x28mm FN Ammunition"

The 5.7 x 28 mm cartridge was designed for pistols, submachine guns, personal defense weapons and carbines.  It was developed by Fabrique Nationale Herstal (FN Herstal), Belgium and primarily marketed to European militaries for use by vehicle crewmen, artillery personnel and combat engineers and as a military and law enforcement round that has more effective performance against modern body armor when compared with traditional pistol caliber ammunition. The ammo for the civilian market WILL NOT penetrate Level IIA and Level IIIA Kevlar, Kevlar 129 or Spectra vests, contrary to popular myth put originated by misguided politicians and gun control advocates trying to advance their agenda.  Ballistically, the 5.7 round is very similar to the .22 WMR (Winchester Magnum Rimfire) round.

 The 5.7x28mm rounds are bottle-necked cartridges with .224 inch (5.7 mm) bullets and come in several versions.  5.7 ammunition is characterized by high muzzle velocity and low recoil, and were designed to be more effective against modern body armor when used in pistols, submachine guns, or carbines."

Added: At least 3 commenters at outdoors fan site,, say FN 5.7mm gun and ammo used at UAH weekend shooting were likely stolen, "odd caliber for drive-by shooting": 

"FN 5.7 - An odd caliber for a drive by shooting"

"My guess is that the gun (and ammo) are stolen."

"Yes, likely stolen - and fired until the magazine in it when taken was emptied, then tossed."

 "My guess is straw purchase or stolen."