Shooting Illustrated:

The simplest, yet most important, difference between the two cartridges is their respective pressure limits. The .223 Rem. cartridge is held to a lower pressure than 5.56 NATO. Some of the testing methods to determine these actual pressures can be confusing, as both cartridges have been tested by the ballistic authorities (read CIP and SAAMI) in the same 5.56 mm chamber, and the resulting data will appear to be nearly equal. However, because of the dimensional variations in the distance between the case mouth and the beginning of the rifling, trying to fire 5.56 NATO ammunition in a .223 Rem. chamber is, simply put, just a bad idea.

The reverse is not true. It is, and always will be, safe to shoot .223 Rem. ammunition in a chamber marked for 5.56 NATO. Commit that idea to memory, and you’ll never get in trouble. The pressures that a 5.56 NATO cartridge can generate are too high for the .223 Rem. chamber, and that is based primarily on the leade dimensions. If you feel that the ability to shoot 5.56 NATO ammunition out of your .223 Rem.-chambered rifle is paramount, take that rifle to a competent gunsmith to have the chamber reamed out to handle 5.56 NATO ammunition.

That chamber dimension for the 5.56 NATO is, in fact, slightly larger than the chamber for the .223 Rem.—in order to have the smoothest feeding and ejection, even with a dirty weapon, to best serve as a battlefield implement—but it is the leade dimension that makes the biggest difference. Leade is defined as the area from the bullet’s resting place before firing to the point where the rifling is engaged. The shorter the leade dimension, the faster the bullet will engage the rifling, and the faster the pressures can rise to a dangerous level.

The most interesting thing about the article is that there is a throw-down in the comments over whether the author is perpetuating the alleged “myth” that 5.56mm cases have thicker walls and therefore less volume, leading to the higher pressure.

John Farnam at Ammoland:

After decades of piously assuring us the 5.56×45 round was “adequate” for military purposes, despite mounting complaints (unsatisfactory range and penetration), dating back to Vietnam, the Pentagon has apparently finally changed its mind.

In spite of a dreary series of failed “wonder bullets” that have, every few years, come forth to “upgrade” the 5.56 round, faith that the 5.56 can ever be “adequate” is fading!

Just as the Marines are buying the HK 416 (M27), a gas-piston AR (in 5.56×45 caliber), to replace aging M4s, Congress and the Army are putting the breaks on that project.

After fifty years of pointless hope that the 5.56×45 round might really be “adequate,” a new, bigger military caliber may now be about to make its debut!

When the AR (in 5.56×45 caliber) first reared its head, and garnered the attention of then Secretary of Defense McNamara, it was slated to gradually replace only the M1 Carbine, never the M1 Garand, later the short-lived M14.

The M1 Carbine, manufactured by the millions during WWII, was originally intended only for rear-area defense and police actions. It was never intended to be a front-line, battle rifle, although it eventually found its way into every corner of the campaign during WWII and Korea.

When I was in Vietnam in 1968, M1 Carbines were still around in large numbers. I saw (and used) plenty of them.

Yet, the AR (in 5.56×45 caliber) somehow eventually became the main, battle rifle of all US Forces, and remains in that status to this day. This, despite continuous misgivings about its adequacy that have been desperately voiced since Vietnam.

Up until now, the Pentagon as assured us that these qualms about adequacy were all in our imaginations!

That is apparently about to change.

Of course, the Pentagon will never admit they’ve been wrong all this time. They’ll simply say “It’s time to move on.”

It was time to move on fifty years ago!

You can color me unimpressed with John’s analysis.  First of all, nothing is going to change because Amerika is flat broke and printing money like there’s no tomorrow.

Second, the only real need for caseless ammunition is so that women can be sent into combat.

Third, there is nothing wrong with the 5.56mmX45.  That’s the real myth here.

The 5.56mm round has killed scores of enemy fighters (hundreds of thousands, perhaps millions) in Vietnam, Africa, Iraq, Afghanistan and South America.  It doesn’t need to be replaced, and it did just fine for my son in Iraq.

There are exceptions, of course.  He once told me of a time when he had to shoot an insurgent with a nine-round burst from his SAW, only to see the fighter keep coming at him.  It took a grenade to stop him.  He also told me that he and other Marines had to continue the fight with insurgents (foreign fighters) who had lost limbs and continued to shoot or fist fight.

Those kinds of fighters are ideologically motivated and doped up on epinephrine and morphine.  They tested them and learned that information after the fact.  It would take a .50 Sasser to bring someone like that down with one shot.

The better option is to teach Soldiers to shoot, uphill and down, at distance, and supplement their ranks with a designated marksman who shoots something larger than the 5.56mm or employ a crew served weapon.  Each weapon system has its purpose, and there isn’t a do-everything gun.  If anyone tells you that, he’s lying.

On the other hand, if they do actually replace the 5.56X45, I’ll just grin and nod and say, “Good.  That’s just more for me.”