The two most common AR 15 calibers on the U.S. market are the .223 Remington and the 5.56×45mm NATO.

Eugene Stoner developed the .223 Remington, based on the .222 Remington sporting cartridge, in cooperation with Sierra Bullets.

In September 1963, the U.S. military type-classified the .223 Remington cartridge, using a 55-grain bullet, as “Cartridge, 5.56mm Ball, M193.” 

In 1964, Remington introduced the .223 cartridge to the sporting market.

Several companies now manufacture sporting, and tactical carbines and rifles chambered in both cartridges.

But which round is more suitable for your purposes?

556 ammo VS 223 ammo: DIFFERENCES

The 5.56mm NATO and .223 Remington cartridges are externally identical except for the headstamp.

As a result, gun owners often ask, “Is it safe to fire .223 Remington ammunition in a rifle chambered in 5.56mm?” 

According to SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute), the answer is yes.

As 5.56mm chambers can vary from one rifle to another, SAAMI recommends consulting the owner’s

manual or contacting the manufacturer for verification. 

However, it is not considered safe to fire 5.56mm ammunition in a 223 rifle.

A barrel marked “.223 Remington” has a shorter throat or chamber leade, which may cause an increase in chamber pressure when firing 5.56mm.

The throat or leade is the part of the barrel that tapers toward the rifling ahead of the chamber.

The maximum chamber pressure that SAAMI recommends for the .223 Remington is 55,000 psi (pounds per square inch),

compared with 62,000 in 5.56mm. On the other hand, while safe, firing .223 Remington in a barrel marked “5.56mm,”

or some variation may not be accurate or reliable.

HYBRID CHAMBERS

Despite the name, the .223 Wylde is not a cartridge but a chamber specification.

The .223 Wylde chamber allows you to fire both .223 Remington commercial ammunition and 5.56mm NATO loads

in the same rifle without compromising accuracy or safety.

.223 VS 5.56

If you’re in the market for a rifle chambered in one of these cartridges, you may be wondering which you should select.

A 223 vs 556 comparison can help you make an informed decision. 

AVAILABILITY AND VARIETY

A rifle chambered in 5.56mm provides increased access to military surplus ammunition and new military loads.

A .223 rifle allows you to take advantage of ammunition for hunting, home defense, and competitive target shooting.

Both .223 and 5.56 cartridges are widely available — supply shortages notwithstanding — so you should always be able to find ammo. 

Standard 5.56mm bullet weights range from 55 to 77 grains, whereas .223 bullets may be as light as 40 grains or as heavy as 90.

ACCURACY

Accuracy depends on several factors, such as the load, the rifle, the sighting system, and the shooter. Both 5.56mm and

.223 Remington ammunition have the potential to be highly accurate.

However, you also need to consider the rifling twist rate of your barrel, especially

if you use both cartridges interchangeably in the same weapon. 

55–60 GRAINS: 1:12

When the 55-grain 5.56mm M193 cartridge was adopted, the standard rifling twist rate was 1:12.

This twist rate is suitable for stabilizing lightweight bullets weighing between 55 and 60 grains.

62–90 GRAINS: 1:7

When the U.S. Army adopted the M855, it specified a rifling twist rate of 1:7 to stabilize the new 62-grain bullet.

The 1:7 twist rate is suitable for stabilizing bullets as heavy as 90 grains and is especially useful for

75- and 77-grain bullets when fired in carbines and pistols.

55–77 GRAINS: 1:9

1:9 is a general-purpose twist rate that can stabilize bullets as light as 55 grains or as heavy as 77 grains. 

HUNTING

For hunting, whether deer or varmints, the .223 Remington is the superior choice.

Lightweight, frangible or expanding bullets are more efficient for killing game animals.

In addition, non-deforming rifle bullets are illegal to use in several jurisdictions, as they have

less wounding capability. 

.223 FIOCCHI 40-GRAIN V-MAX

Loaded with a 40-grain polymer ballistic tip, the Fiocchi V-Max load has a listed velocity of 3,650 ft/s.

The high velocity, lightweight bullet, and polymer tip ensure reliable expansion and a high ballistic coefficient.

This round is ideal for hunting varmints, including coyotes; however, its flat trajectory is also suitable for long-distance shooting.

.223 HORNADY 50-GRAIN FULL BOAR 

For hunting medium game like whitetail deer, Hornady’s Full Boar .223 load consists of a 50-grain monolithic

GMX copper-alloy bullet with a polymer ballistic tip.

The monolithic construction ensures deep penetration and high weight retention, while the polymer tip promotes expansion.

According to the company, in a 24” test barrel, this load achieves a velocity of 3,335 ft/s. 

SELF-DEFENSE

Commercial .223 ammunition typically consists of lead-cored projectiles with jacketed spitzer,

exposed lead, or ballistic tips for hunting and recreational and competitive target shooting. 

As a result, .223 expanding ammunition is generally more suitable for home defense

than non-deforming full metal jackets (whether .223 or 5.56).

However, 5.56mm OTM bullets, optimized for use in carbines and pistols, are also effective for this purpose.

5.56MM MK318

The United States Marine Corps adopted the Mk318, an alternative to the U.S. Army’s M855A1, in 2010.

The Mk318 Mod 0 and 1 use a barrier-blind 62-grain OTM bullet to achieve reliable

fragmentation in soft tissue even when fired from a 10.5” barrel.

This represents a significant improvement relative to the M193 and M855.

5.56MM MK262

Black Hills Ammunition developed the Mk262 Mod 0 and Mod 1 for the SOCOM special-purpose rifle (SPR).

Loaded with a 77-grain Sierra MatchKing OTM bullet, the Mk262 has demonstrated superior accuracy and

terminal performance compared with the M855. As with the Mk318, the Mk262 is effective when fired

from pistol- and carbine-length barrels.

.223 55-GRAIN HORNADY CRITICAL DEFENSE FTX 

Hornady’s Critical Defense FTX ammunition uses a 55-grain bullet with the company’s Flex Tip,

which promotes expansion at relatively low velocities and prevents the nose cavity from becoming clogged.

Critical Defense rounds use low-flash propellants and nickel-plated casings for low-light chamber checks.

The polymer tip increases feeding reliability.

ARMOR/BARRIER PENETRATION

If you expect to face armored threats, whether as a private citizen or in an official capacity,

5.56mm M855 and M855A1 ammunition are ideal.

In addition, if you foresee the need to penetrate sheet metal and windshield glass, these loads are also more penetrative

than FMJ lead-cored ammunition. 

M855/SS109 (62 GRAINS)

The M855/SS109 cartridge is the 5.56mm NATO cartridge. The M855 uses a 62-grain

semi–armor piercing full metal jacket boat tail (FMJ-BT)

containing a 10-grain, .182-caliber hardened steel penetrator ahead of a lead core.

The U.S. Army adopted this round in 1982 to replace the M193 for use in the M249 SAW (Squad Automatic Weapon). 

M855A1 ENHANCED PERFORMANCE ROUND

The U.S. Army adopted the M855A1 in 2010 as a replacement for the M855. The M855A1 uses

a heavier hardened penetrator and a copper slug.

The new round exhibits more consistent terminal performance than the M855, and the A1 is

optimized for use in the M4 carbine and other short-barreled weapons.  

A RIFLEMAN NEEDS A SIDEARM

Both the 5.56mm and .223 cartridges are effective for a variety of applications,

both practical and recreational.

However, you should also have a sidearm whether you carry a rifle for duty, as a hunter, or for defense.