The 5.56 or 5.56×45mm NATO is a rimless bottlenecked intermediate cartridge family developed in the late 1970s in Belgium by FN Herstal.
Some of the military and tactical rifles chambered in the 5.56 cartridge are the M-16 and AR15, M4, HK MR556, IWI Tavor, and the FN SCAR 16:
HK MR556A1 556MM HK416 RIFLE 5.56mm NATO 223REM. Available in California from Cordelia Gun Exchange!