The .223 Remington is a commonplace rifle cartridge, originally developed for the M16 service rifle of the United States Army. Adopted with slight modifications by the military as the 5.56x45mm military round, the original .223 ammunition gained widespread popularity among the civilian population, specifically in the Colt Rangemaster, of which the round had been a standard of since 2051. The common variant has a full metal jacket, offering good penetration without sacrificing damage. Its wide availability makes it a popular choice for wastelanders, due to the number of weapons that can accept the round.
The .223 is affected by the common executable bug, where the ammunition modifiers do not work. This has been fixed in Fallout 2.
↑Vault Dweller's Survival Guide p.5-15: "A longer ranged, and more powerful, firearm. The Rangemaster is your basic, solid rifle. It uses the .223 caliber rifle round, a standard for over 110 years. The .223, combined with a 1:10” twist, gives good accuracy at range and solid knockdown capability."
↑Vault Dweller's Survival Guide p.5-17: "Rifle caliber ammunition. The FMJ stands for Full-Metal Jacket, which describes the bullet that is used in the round of ammo. A FMJ bullet is very tough, and has pretty good penetration without sacrificing good expansion. In other words, it‘s an average round."
↑Fallout Official Survival Guide p.119: ".223 Full Metal Jacket (FMJ): This is the same ammo used in the late 20th century by various military rifles, including the M-16. A good, basic, hard-hitting round."