Why 5.56 / .223 is BOTH the Best and Worst AR-15 Cartridge

You’re probably aware there’s a raging debate about the 5.56 / .223 cartridge’s “stopping power” these days.

Some say the cartridge is only suitable for varmints, others think it’s the greatest close range combat round ever invented.

As usually is the case, the truth lies somewhere in the middle.

The 5.56 can be a very effective cartridge, which doesn’t mean it always is an effective cartridge.

First, a (Highly abbreviated and little known) history of the 5.56. 

After WWII , the US military wanted a smaller lighter cartridge for it’s troops.  That eventually led to the creation and adoption of the M14.

The US Air force didn’t like the M14 because it was heavy, and weight is at a premium on airplanes.  So in 1964 they instead opted for the “Space Age” M16 because it – and it’s 5.56 ammo – were lightweight.

The US Air Force almost NEVER engages in soldier-to-soldier combat.

To this day, I think they only have one Combat MOS (Military Occupational Specialty).  The M16 and it’s 5.56 cartridge was only going to be used at close range for security guards, military police, etc.

I would assume also as a defensive weapon in case of your aircraft was shot down.

The M16 / 5.56 was NEVER designed or intended to be a frontline main combat weapon.

Before the M16 (in 5.56) was adopted, two test batches were sent to Vietnam.  The first was only 10 rifles in October of 1961.  The second batch was a 1000 rifles sent in 1962.

In the CLOSE RANGE combat of Vietnam, the tiny 5.56 bullet produced wounds were so impressive, that pictures of them stayed classified until the 1980s.  This isn’t surprising because the 5.56 was designed for close range combat.

How the 5.56 Does it’s Damage

Spoiler Alert: it’s primarily from fragmentation.

The research that created this picture was originally done by Martin Fackler

As you may have gathered from the picture above, fragmentation is dependent on velocity.  At higher velocities, all those tiny fragments destroy a LOT of tissue causing a serious wound.

However, that only happens at high velocities.

Here is the original military issue 55 grain 5.56 bullet’s ballistics out to 200 yards.  I think you’ll see the problem.

The picture above shows that fragmentation gets spotty around / below 2600 FPS.  The 5.56 bullet is going just over 2600 fps at 150 yards.

So after 150 yards the 5.56 is almost out of steam as far as fragmentation goes.

The bullet can still kill if it hits something vital.  But it’s a small bullet and doesn’t create an effective wound unless it yaws and tumbles (more on that in a second)

After the military adopted the 5.56, they decided to create a “better” bullet.

They ended up with the 62 grain “green tip” light armor piercing bullet (military designation is M855)  Later, they decided to shorten the 20″ barrel of the M16 to the 14.5″ barrel of the M4.

Shorter barrels mean less velocity.

So here’s the velocity of the new 62 grain Green tip.

See the problem?

At 100 yards, the “improved” bullet is only going 2600, which is where fragmentation gets spotty.  The bullet lost 50 yards of effective range by “improving” it.  Worse, the new bullet is almost resistant to yawing and tumbling, which causes fragmentation.

What is Yawing? 

I’m glad you asked.

It simply means the bullet starts to rotate so that it’s going sideways through the target.

Bullets are heavier in the back than front because of their shape.  This means the bullet wants to turn around in the air.  Rifling makes the bullet spin to prevent this while traveling to the target.

This is yawing without any fragmentation. (which usually happens before the bullet has turned fully sideways)

If the bullet strikes on an “off angle” like so:

It can yaw and tumble very quickly.  However early yawing only happens about 20% of the time.

Even worse, about 20% of the time 5.56 bullets will pass completely through the target without yawing or tumbling at all.  This is usually happens when the bullet strikes “head on” and there is nothing to destabilize it. It will often drill a tiny little hole like this:

That does very little damage unless the bullet hits a very vital organ (brain, heart, spinal cord etc)

The other 60% of the time, only God know when the bullet will start to yaw and tumble.  It might be after a few inches, in might be after a foot or more.

If it yaws too late, then very little damage is done.

Important: a 5.56 bullet will only fragment if it yaws.

So even within that 100-150 yards of effective range, the bullet might do almost no damage at all.

Of course, it might make a wound that looks like a blender and food processor got into a war.  There’s just no way to know which it will do.

If you wondered why the reports about 5.56 lethality are so inconsistent, that’s why.  The bullet’s performance is inconsistent.

That said: it’s a problem with the bullet, NOT the cartridge.

The military has recently adopted a 77 grain Match bullet. It extends fragmentation range somewhat, and yaws more often.  However, it’s still not consistent in yawing, and isn’t very effective beyond about 200 yards.

A MUCH better option is a quality Expanding bullet designed for hunting.

Quality 5.56 / .223 Hunting ammunition does not have the yawing problem.  It also has a much longer effective range because it will expand at a much lower velocity.  Typically around 1800 fps.

The defensive / combat ammo in my AR-15 (.223 Federal Fusion MSR) is effective to around 350 yards from my 16″ barrel.

BTW, a great resource for picking great defensive/combat ammo is here: The Best choices for Self defense ammo

Many countries have outlawed the use of Military style FMJ ammo for hunting because it’s likely to wound, but not kill an animal.

Many states have outlawed hunting deer-sized game with bullets smaller than 6mm. (like 5.56)

Why?

Because any smaller and you might not have enough bullet to cleanly and ethically kill game.

The military did some extensive testing in the 1920s and early 1930s and decided that 120-130 grains was the ideal bullet weight for combat.  They also decided that between 6.5mm and 7mm was the ideal bullet diameter.  (More info in my General Purpose Combat Cartridge Article)

The 5.56 is smaller in diameter and has half the weight of the “ideal” combat bullet. It’s a problem, but the 5.56 can be VERY lethal at close range with good bullet selection.

One of it’s biggest virtues is the very low recoil.  That allows for fast, accurate fire in close quarters.

And, you can’t miss fast enough to win a gun fight.

So remember, the Military’s 5.56 is inconsistent because of the bullet.  That’s the main problem.  If you you pick a quality expanding bullet, then the 5.56 can be effective on humans out to 300-400 yards.

For hunting, it’s iffy.

Yes a good expanding bullet can cleanly kill deer sized animals.  I’m sure plenty of people do it.  But the margin for error is low.  Too low for me.  In an AR-15, I would prefer the 6.5 Grendel cartridge.

The 5.56 does have some very cheap plinking ammo.  That makes practice easy and being a good shot will trump the caliber or cartridge chosen any day…

…Because you can’t miss fast enough to win a gun fight or fill a hunting tag.