Per SAAMI specifications: The 5.56 and .223 Remington cartridges are similar, but the chambers of the guns and test barrels they are evaluated in are different. One of the key differences is the length of the throat or leade on the 5.56 chamber compared to the .223 Remington. It’s almost twice as long in the 5.56 chamber versus the .223 Remington chamber.
Typically, the 5.56 round is loaded to a higher velocity and pressure level than the .223 Remington.
Firing the 5.56 round in a .223 Remington chamber with the shorter leade can dramatically increase chamber pressures.
Therefore, we recommend against firing the 5.56 round in a .223 Remington chamber, unless the gun is marked with both cartridge designations such as our Model 7615 Patrol Rifle.