|The following is based on S.T.A.L.K.E.R.: Clear Sky cut content and has not been confirmed by canon sources.|
.223 FMJ or .223 Remington (referred to in game code only as ammo_223_fmj) is a high-caliber explosive ammunition which was partially included in S.T.A.L.K.E.R.: Clear Sky.
The .223 Remington cartridge is a sporting round with dimensions that are nearly identical to the 5.56x45mm SS109 cartridge. The differences between the two rounds are so slight that it is safe to use .223 rounds in a weapon chambered for 5.56x45mm rounds, though the reverse is not true. It is one of the most commonly used rifle cartridges in the United States, used in both bolt-action rifles as well as semi-automatic weapons such as the AR-15.
.223 FMJ rounds are only used by the ".223 Pistol" (a .223 caliber version of the Heckler & Koch USP Compact) and the Bozar; neither weapon is available in the game without the use of cheats or modifications. Both weapons are Easter egg references to weapons of the same name from the Fallout series of games.
.223 FMJ rounds only exist in Clear Sky and are not normally found in the game. The ammunition remains active in existing game code but is not carried by any Stalkers or available for purchase from any merchants, it can be found only in Modifications. Ammunition boxes use the same visuals as 7.62x54mm 7N14 rounds both in-game and in the player's inventory. All text for the ammunition, both name and description, are blank, displaying only the variable names used (e.g. "ammo_223_fmj").
.223 rounds are the second highest damaging bullet ammunition in the game, behind .45 ACP Hydroshock rounds. They are second in armor piercing capability only to Gauss Rounds. These two advantages combined with the ammunition's explosive properties make it one of the most lethal rounds present in game code.