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.
The standard type of 5.56mm ammunition offers good performance to the shooter. Popular among civilians and soldiers alike, it is by far one of the most popular rounds in the wastelands. It is essentially the workhorse of the Guns-skill weapon class. Common, cheap, and utilized by numerous weapons, this round and its variants will stay with the player throughout the game.
The standard type of 5.56mm ammunition, offering good performance to the shooter. Popular among civilians and soldiers alike, it is by far one of the most popular rounds in the wastelands. It can generally be used interchangeably with its parent round, the .223 Remington, due to the same casing, though some weapons might not be rated for the military-grade ammo. In such a case, the 5.56x45mm round can cause dangerous buildup of pressure inside the receiver.
A variant of the 5.56mm ammunition, designed specifically to defeat enemy body armor. The softer metal jacket covers a dense penetrator core, while the cartridge is loaded with a larger powder load, for increased muzzle velocity. However, improved performance against armored targets leads to impaired performance against softer targets, as the higher velocity reduces the amount of kinetic energy transferred to the target on impact.
A variant of the 5.56mm round sitting on the opposite side of the spectrum from armor-piercing ammunition, hollow point bullets feature a pit or a hollowed out shape at the tip of the bullet. As such, the bullet expands upon impact, delivering more kinetic energy to the target and disrupting tissue more extensively. However, this same property also leads to dramatically reduced penetrative power, as the bullet will be stopped by the armor more easily due to its properties.
Although it might seem that ammunition remains generally the same, it is not so. Ammunition is constantly refined and new variants are introduced, replacing the old ones. The latter are either issued to second line units or sold on the market at discount prices. The 5.56mm surplus ammunition is exactly that, offering slightly improved damage at the expense of causing more wear to the firearm. It is available cheaply in bulk, however.
A variant introduced by the Gun Runners into the market, 5.56mm match grade rounds are manufactured with extremely tight engineering tolerances and a high level of quality control. The resulting cartridge is perfectly suited for competitive shooting due to its superior ballistic performance, resulting in improved damage and much greater accuracy at range.
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. It can be bought in bulk from vendors.
↑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."