What is 556 caliber?

Answer 1

The 5.56x45 cartridge is dimensionally similar to a .223 Remington. However, the two are not the same. A 5.56 chamber can safely fire .223 rounds, but a dedicated .223 chamber can't necessarily fire 5.56 cartridges safely, given the different angle of the cartridge neck, and the higher chamber pressures generated by the 5.56 cartridge over the commercial .223 round.