The 5.56×45mm NATO (5.56, 5.56x45mm, or 5.56 NATO) cartridge is one of the standardized cartridges for the nations in the NATO organization. It is derived from the .223 Remington cartridge, with which it shares many similarities; the two cartridges are visually identical.
The 5.56 NATO cartridge is visually nearly identical to the .223 Remington cartridge, with the exception of the casing headstamp, which usually bears the NATO insignia. It can be fired in some of the same weapons. The 5.56 can use the same projectiles as .223, as well as projectiles developed specifically for 5.56.
The cartridge uses a small rifle primer, and is loaded to a pressure that would be considered excessive in a .223 chamber, however - the .223 chamber has a shorter and sharper leade compared to the 5.56 chamber, as well as less effective freebore. Firing this cartridge in a .223 chamber will cause the pressure to spike due to the projectile engaging the rifling sooner than it would in a 5.56 chamber.  Use caution when firing 5.56 NATO cartridges in a weapon suspected to have a .223 chamber; common symptoms are popped primers and gas staining on the primers. Contrary to popular belief, the 5.56 NATO brass cases do not have significantly reduced case capacity due to the thickness of the case; there is no discernible difference, though the capacity will vary slightly due to manufacturing tolerances.
The M193 projectile will fragment reliably so long as its flight velocity remains at or above 2700 feet per second; the M855 projectile will fragment reliably above 2500 feet per second.
The 5.56 NATO bullet is actually 5.71mm (.224 inches) in diameter, and is best fired in a barrel with a 1:8 rifling twist; however, most rifles use either a 1:7 or 1:9 twist. Some older rifles use a 1:12 or 1:14 twist.
The maximum pressure is about 63,000 PSI; the 5.56 proof load is 70,000 PSI.
The 5.56x45mm NATO has several variations and designations, both military and non-military.
This cartridge has a full metal jacket 55-grain projectile. The bullet does not bear any special markings. This cartridge is loaded to a pressure of 52,000 psi.
This cartridge is a tracer, ballistically matched to M193 ammunition. Its tip is painted red.
This cartridge has the same projectile as the M193; however, this is a proof round, loaded to very high pressures to conduct high pressure tests (HPT) on barrels and bolts. This round is loaded to a pressure of 70,000 psi.
This cartridge is a dummy round. It is marked by several indentations in the casing. There is no primer, and no propellant is loaded.
This cartridge is a blank. It is identified by the violet tip; older M200 ammunition had a white tip.
This cartridge has a 62-grain full metal jacket projectile, designed to offer better penetration against steel. It has a steel penetrator inside the projectile. This cartridge is identified by the green paint on the tip of the bullet, and is loaded to a pressure of 55,000 psi.
This cartridge has a 62-grain FMJ projectile, but with no lead. It is loaded to a much higher pressure (63,000 PSI) than regular M855, which will lead to greater wear and tear in any 5.56 weapon it is fired in. This cartridge has a steel tip.
This cartridge is a tracer, ballistically matched to M855 ball ammunition. The bullet is very long due to a large load of tracer compound inside the bullet, but the OAL (overall length) of this cartridge is identical to other 5.56 NATO loads. The bullet is painted orange at the tip, and weighs in at 64 grains. M856 does NOT have a steel penetrator.
This cartridge (nomenclature M862 SRTA (Short-Range Training Ammunition)) is designed exclusively for training. It requires the use of a special bolt in the AR-15, and closely replicates the ballistics of service ammunition out to 25 meters.
This cartridge has a 52-grain armor piercing projectile, composed of a tungsten inner core with steel sheath, lead filler and gilding metal jacket. The bullet is painted black at the tip.
This cartridge is a tracer, ballistically matched to M995 AP ammunition. The bullet is painted dim violet at the tip.
Mk 255 mod 0
This cartridge has a 62-grain bullet, designed for reduced ricochet and limited penetration. It is marked by gray paint on the tip of the bullet.
This cartridge has a 77-grain open-tip projectile. It shoots with a flatter trajectory and has a better terminal ballistic profile than most other 5.56 cartridges. This cartridge is loaded to a pressure of 58,700 psi. There are two variants of this cartridge: Mk 262 mod 0, which has no cannelure on the bullet, and Mk 262 mod 1, which does have the cannelure.
Mk 318 mod 0
This cartridge has a 62-grain projectile. It fragments quite well on impact while retaining adequate penetration due to its design; it has an all-copper bullet with an open tip, and a solid body (allowing the rear part of the bullet to penetrate beyond the original wound, and through some barriers).