.223 round

The .223 round (.223 Remington) is a type of ammunition in Fallout, Fallout 2, and Fallout: New Vegas. The military variant, the 5.56x45mm military round, appears in Fallout 3, Fallout: New Vegas, and Fallout Tactics. The two ammunition variants can be used interchangeably.

Background
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. 

.223 FMJ
The basic variant of the .223 round offers good penetration and expansion, without excelling in either. This commercial cartridge has identical dimensions to the military round, though it is loaded with less powder, leading to lower internal pressures. Though this lowers damage, it leads to less wear on the weapon.