|For the Fallout, Fallout 2 and Fallout: New Vegas ammunition round, see .223 round.|
|This is an overview article, focusing on background information and cross-game comparisons.|
The 5.56x45mm round is a rifle cartridge derived from the .223 Remington, originally developed for the M16 service rifle of the United States Army. It is a rimless, bottleneck cartridge that offers tremendous wounding potential under the right condition. Its performance and popularity among military forces endures despite the Great War: By 2287, it has served Americans for over two centuries, from the United States Army to the New California Republic Army.
Ammunition. 5.56mm caliber. The 5.56mm round is a standard intermediate caliber rifle cartridge used in most military, hunting and sporting rifles with almost the same external dimensions as the .223 round, but while the external case dimensions are very similar, the 5.56mm and .223 differ in both maximum pressure and chamber shape.
5.56mm, armor piercing
Ammunition. 5.56mm caliber, armor piercing. Armor piercing rounds are designed for punching through armor plating, for this purpose they have a greatly strengthened case with a specially hardened and shaped nose. Due to the high muzzle velocity and penetrative properties of the round, damage to the target is slightly reduced.
5.56mm, hollow point
Ammunition. 5.56mm caliber, hollow point. Hollow point bullets mushroom and break up on impact, causing massive trauma on fleshy, un-armored targets. However, this also means they break up when they hit things like armor plate, drastically reducing the weapon's penetrating power.
Ammunition. 5.56mm caliber, surplus. Surplus ammunition is poor grade ammunition made in larger quantities by using corrosive materials in their construction. The resulting process is a slightly more damaging round but at the cost of more regular maintenance on a firearm due to the highly damaging corrosive materials used in the round's construction.
New ammunition added with the add-on, Gun Runners' Arsenal. Match ammunition is precision-made so that every bullet has the same powder, shape, and weight. Match ammunition is often used in tournaments, sight alignment, and sniping. This ammunition type gives a slight damage boost and a significant reduction in the weapons spread.
Behind the scenes
The real-life equivalent is the 5.56x45mm NATO intermediate rifle cartridge, fired by most assault rifles and light machine guns used by NATO countries. The .223 Remington is the 5.56's parent cartridge.
- 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."