How can I determine if chamber is 5.56 or .223

[andrew]

Several years ago I built an AR15 on a Rock River Arms 5.56 marked lower. I'm afraid that I didn't pay as much attention to the upper, and am not sure if it is 5.56 or .223. There are no markings indicating caliber, that I can see.

All I have ever fired out of it are .223 which I understand is OK out of either chambering, but not the other way around. Is there an easy way to determine chambering?

[LivoniaDan]

Really... I don't have the faintest idea... but.....
.223 calculates out to 5.66... a tenth of a millimeter different....
but that might be as good as a mile... I just don't know.....

Not even sure why I answered here... just wanted to... I guess....

[RSF]

get gauges from brownells the only issue you may have is if shooting milsurp ammo... you can get that fixed by using a ned's chamber reamer to fix it up few twist and your good to go .

[TFin04]

Interesting. I always thought they were the same round one just standard and one metric.

Obviously I don't yet own an AR and haven't read enough about building one.

Can anyone give me the layman's terms of the difference between the two?

Thanks!

[Mike Kelly]

Several years ago I built an AR15 on a Rock River Arms 5.56 marked lower. I'm afraid that I didn't pay as much attention to the upper, and am not sure if it is 5.56 or .223. There are no markings indicating caliber, that I can see.

All I have ever fired out of it are .223 which I understand is OK out of either chambering, but not the other way around. Is there an easy way to determine chambering?

I'd call Gander Mt. and see if they have anyone there with the right gauges. It's cheaper than buying the gauges and stuff yourself.

[karcent]

I would stay far away from Gander Mountain if I needed a gunsmith.
A headspace gauge probably won't reveal anything since the
5.56nato and .223rem are identical externally.
The difference is in the internal pressure that the ammunition is loaded to.
A 5.56nato round operates at higher pressure than the .223rem sporting round which causes the brass to expand more which can cause extraction problems.

Check the top of your barrel between the front sight base and the muzzle. If it's marked, that's where it's likely to be. Not all barrels are marked.

[andrew]

I would stay far away from Gander Mountain if I needed a gunsmith.
A headspace gauge probably won't reveal anything since the
5.56nato and .223rem are identical externally.
The difference is in the internal pressure that the ammunition is loaded to.
A 5.56nato round operates at higher pressure than the .223rem sporting round which causes the brass to expand more which can cause extraction problems.

Check the top of your barrel between the front sight base and the muzzle. If it's marked, that's where it's likely to be. Not all barrels are marked.

The externally identical part is hurting my head. If the rounds are exactly the same dimensions, what causes the extraction problems at higher pressure? Something to do with headspace?

[TFin04]

A 5.56nato round operates at higher pressure than the .223rem sporting round which causes the brass to expand more which can cause extraction problems.


So with that information, one looking for an all around AR15 should buy one chambered in 5.56 in order to shoot either ammunition without problem?

I assume the 5.56 is also considered more powerful? Higher pressures usually mean higher velocity.

[keyjockey]

A reprint from.......... I can't remember where.



Q. What is the difference between 5.56×45mm and .223 Remington ammo?

In the 1950's, the US military adopted the metric system of measurement and uses metric measurements to describe ammo. However, the US commercial ammo market typically used the English "caliber" measurements when describing ammo. "Caliber" is a shorthand way of saying "hundredths (or thousandths) of an inch." For example, a fifty caliber projectile is approximately fifty one-hundredths (.50) of an inch and a 357 caliber projectile is approximately three-hundred and fifty-seven thousandths (.357) of an inch. Dimensionally, 5.56 and .223 ammo are identical, though military 5.56 ammo is typically loaded to higher pressures and velocities than commercial ammo and may, in guns with extremely tight "match" .223 chambers, be unsafe to fire.

The chambers for .223 and 5.56 weapons are not the same either. Though the AR15 design provides an extremely strong action, high pressure signs on the brass and primers, extraction failures and cycling problems may be seen when firing hot 5.56 ammo in .223-chambered rifles. Military M16s and AR15s from Colt, Bushmaster, FN, DPMS, and some others, have the M16-spec chamber and should have no trouble firing hot 5.56 ammunition.
Military M16s have slightly more headspace and have a longer throat area, compared to the SAAMI .223 chamber spec, which was originally designed for bolt-action rifles. Commercial SAAMI-specification .223 chambers have a much shorter throat or leade and less freebore than the military chamber. Shooting 5.56 Mil-Spec ammo in a SAAMI-specification chamber can increase pressure dramatically, up to an additional 15,000 psi or more.
The military chamber is often referred to as a "5.56 NATO" chamber, as that is what is usually stamped on military barrels. Some commercial AR manufacturers use the tighter ".223" (i.e., SAAMI-spec and often labeled ".223" or ".223 Remington") chamber, which provides for increased accuracy but, in self-loading rifles, less cycling reliability, especially with hot-loaded military ammo. A few AR manufacturers use an in-between chamber spec, such as the Wylde chamber. Many mis-mark their barrels too, which further complicates things. You can generally tell what sort of chamber you are dealing with by the markings, if any, on the barrel, but always check with the manufacturer to be sure.

Typical Colt Mil-Spec-type markings: C MP 5.56 NATO 1/7
Typical Bushmaster markings: B MP 5.56 NATO 1/9 HBAR
DPMS marks their barrels ".223", though they actually have 5.56 chambers.

Olympic Arms marks their barrels with "556", with some additionally marked "SS" or "SUM." This marking is used on all barrels, even older barrels that used .223 chambers and current target models that also use .223 chambers. Non-target barrels made since 2001 should have 5.56 chambers.

Armalite typically doesn't mark their barrels. A2 and A4 models had .223 chambers until mid-2001, and have used 5.56 chambers since. The (t) models use .223 match chambers.

Rock River Arms uses the Wylde chamber specs on most rifles, and does not mark their barrels.
Most other AR manufacturers' barrels are unmarked, and chamber dimensions are unknown.




Opinion: In general it is a bad idea to attempt to fire 5.56 rounds (e.g., M193, M855) in .223 chambers, particularly with older rifles.
Fact: SAAMI specifically warns against the use of 5.56mm ammo in .223 chambers. The .223 SAAMI specification was originally made with bolt rifles in mind.
For more see the SAAMI website ammo warning.

5.56 v. .223 Remington specification.
Fact: The different manufacturer's chamber types are listed at length and in great detail

[LivoniaDan]

A reprint from.......... I can't remember where.Your reprint is from HERE (http://www.ammo-oracle.com/#diff ) I think... Also has diagrams of chambers.

[andrew]

Thank you all for the explanations and links. I almost feel like I could explain the difference to someone else now. :)