Coupled with ammunition selection, a technical barrel issue must be reviewed. As stated, the AR- 15 /M- 16 system is chambered for the most common patrol rifle caliber, the .223 Remington/5.56 ? .45 mm NATO round. Rifles can be chambered for either .223 or 5.56. The commercial .223 can be fired with no issue in a military 5.56 chamber, but not the reverse.