The .223 round is a type of ammunition in Fallout, Fallout 2, and Fallout: New Vegas. Due to the specific dimensions of the .223 round, it is used as an offshoot of normal 5.56mm ammunition and will fit in any weapon chambered for 5.56mm.
The .223 round is a cartridge with almost the same external dimensions as the 5.56mm, but while the external case dimensions are very similar, the .223 and 5.56mm differ in both maximum pressure and chambering.
Standard .223 caliber ammunition has a lower internal pressure than its military standard cousin, and as such deals less damage to the target. However, this also decreases the stress put on the weapon, resulting in better weapon durability.