5.56 x 45 mm NATO

5.56 mm NATO, designated STANAG 4172, is the standard rifle ammunition for NATO forces. It is derived from, but not entirely interchangable with, the .223 Remington cartridge.

Performance

The 5.56 NATO cartridge with the standard military ball bullet (NATO SS109; M855 specifically in the United States) will penetrate approximately 15 to 20 inches (380 to 500 mm) into soft tissue in ideal circumstances. All spitzer shaped projectiles are prone to yaw in soft tissue. However, at impact velocities above roughly 2700ft/s (or 820m/s), the 5.56 mm projectile will yaw and then fragment at the cannelure. The effectiveness of fragmentation seems to impart much greater damage to tissue than bullet dimensions and velocities would suggest. It should be noted that this fragmentation effect is highly dependent on velocity, and therefore barrel length: short-barreled rifles generate less muzzle velocity, therefore rounds lose effectiveness at much shorter ranges than longer-barreled rifles.

Related Topics:
Cannelure - Barrel - Muzzle velocity

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Separately, Hydrostatic shock is often cited as a primary wounding or incapacitation in high velocity projectiles (like 5.56 NATO). The dynamics and actual effects of Hydrostatic shock are, however, highly controversial.

Related Topics:
Hydrostatic shock

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There has been much criticism of the poor performance of the round, especially the first-round kill rate when using weapons that don't cause the bullet to fragment. Typically, this only becomes an issue at longer ranges (over 100 meters) or with shorter barreled weapons that do not propel projectiles to the critical fragmentation velocity. The 14.5" barrel of the U.S. military's M4 carbine can be particularly prone to this problem.

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The small round simply doesn't do enough damage when unfragmented, especially without a criticals hit, to drop a target reliably. Barrier performance is also relatively poor (admittedly, even 7.62 mm NATO is not particularly effective through vehicles), partly because the light and fast round is easily deflected.

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5.56 NATO vs .223 Remington

NATO specification ammunition is not always safe to fire in .223 Remington chambered rifles (mainly civilian sporting rifles). The chambers of weapons intended to take the NATO round are oversize relative to the civilian weapons to allow for greater variation in production (and hence reliability). By contrast SAAMI-specification chambers, however, are manufactured tighter (for accuracy), and "5.56x45" ammunition at the larger end of the manufacturers' tolerances will be too tight and result in overpressure. This can cause excessive wear, or even cause receivers to blow out. Conversely, civilian .223 ammunition is perfectly safe to use in military rifles.

Related Topics:
NATO - .223 Remington - Chambers - SAAMI

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Additionally, some .223 rifles with relatively slow rifling (a common twist rate is only 1 turn in 12 inches for civilian varmint guns), although they could safely fire the military ammunition, will be less accurate. As a general rule of thumb, a rifle with a 1:12 twist rate is best used for 55-grain (or less) rounds, 1:9 for 70-grain (and below), and 1:7 for heavier.

Related Topics:
Twist rate - Varmint guns - Grain

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Comparison of 5.56 vs 7.62 NATO

5.56 NATO

• Size: 5.56 x 45 mm
• Bullet weight: 3.95-5.18 g
• Velocity: 930-772 m/s
• Energy: 1,700-1,830 J

7.62 NATO

• Size: 7.62 x 51 mm
• Bullet weight: 9.33 g
• Velocity: 838 m/s
• Energy: 3,275 J

The NATO Ball round (U.S. = M855) can also penetrate up to 3 mm of steel while the Armor-piercing variant (U.S. = M995) can penetrate up to 6 mm.

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