223 Rem and 5.56 NATO are the most common chamberings used in the Americas most popular rifle, the AR15.
The .223 Remington and 5.56×45mm NATO cartridges and chamberings are similar but not identical.
While the cartridges are identical other than powder load, the chamber leade, i.e.
distance from the projectile while seated in the case to the rifling is typically shorter
in .223 Remington commercial chambers.
Because of this, a cartridge loaded to generate 5.56×45mm NATO pressures
in a 5.56×45mm NATO chamber may develop pressures that exceed SAAMI limits when fired
from a short-leade .223 Remington chamber