5.56 or .223 ??? what


January 11, 2004, 10:47 AM
I was considering purchasing a new upper for my A2. On the thread it says to specify chamber and it says 5.56 NATO or .223. I thought .223 and 5.56 were the same.. Am I missing something or what?


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January 11, 2004, 10:56 AM
This is an excerpt from a very detailed composition on AR15.com some time ago. I don't have the link but I saved the document:

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 with a longer throat and should have no trouble firing hot 5.56 ammunition. The big difference between the two chambers is in the chamber dimensions. Military M16s have slightly more headspace and have a longer throat area, compared to the Error! Bookmark not defined. .223 chamber spec, which was originally designed for bolt-action rifles. Commercial SAAMI-specification .223 chambers have a much shorter throat, a smaller diameter bullet seat 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 AR manufacturers use the tighter ".223" (i.e., SAAMI-spec and often labeled ".223" or ".223 Remington") chamber, which tends to give you more accuracy but, in self-loading rifles, less reliability, especially with hot-loaded military ammo. Some 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 on the weapon, 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"
Armalite doesn't always mark their barrels.

Opinion: In general it is a bad idea to attempt to fire 5.56 rounds (e.g., M193, M855) in .233 chambers, particularly with older rifles.

Fact: specifically warns against the use of 5.56mm ammo in .223 chambers. The .223 SAAMI specification was originally made with bolt rifles in mind

Q. Which should I be looking for in an AR15, 5.56 NATO or .223 Remington chambers?
This is really a matter of the role for which you plan to use your AR. .223 Remington chambers will give you slightly better accuracy, which is important for a match or varmint rifle. Any loss of feeding and cycling reliability and the restriction against shooting military ammo isn't as important as the accuracy gains for a rifle used in these roles, because for these rifles, accuracy is everything. People who just want to plink or who plan to shoot military ammo (such as most of the cheap surplus ammo available), and especially those who may use their AR as a weapon, should choose 5.56 chambers.

Opinion: Unless you have a reason to seek out .223 Remington SAAMI spec chambers, 5.56 NATO is probably the best solution. 5.56 NATO chambers still can have outstanding accuracy and give you more flexibility in ammo selection

January 11, 2004, 11:31 AM
I now know. Thanks. I think I'm going to go for the .223 instead of a 5.56, first since I am going to buy another upper for accuracy the .223 will be much more appreciated, also I have a 5.56 20" upper now for "fun time".
Thanks, I do appreciate you taking time to give me this valuable information.

Thanks again

January 11, 2004, 11:33 AM
Spieler has given you a great answer here.

I have owned both an AR and a .223 chambered rifle for years and never knew there was a difference in the 5.56 chamber until a few weeks ago when I learned what was posted here under my question.

The big advantage to getting the 5.56 chambered upper is that you can shoot the military surplus ammo (cheap) without worry.

My 5.56 chambered Bushmaster is very accurate so I don't think there is a loss there.

January 11, 2004, 11:38 AM
The saying of you never stop learning comes to mind.... lol What about barrel twist? Which would be better for different barrel lengths? For example. 1:8 or 1:9, or 1:14. which would be better for 16" or 20" or even 24". I would think the more rotation on the bullet would be better in a 24" barrel, but what are your opinions????

Thanks again

January 11, 2004, 12:05 PM
Crow (welcome to THR, btw!),

What is your intended mission for this rifle? If this were your idea of a varmint gun, a slower twist- 1 in 12, say- would be okay. More versatile would be 1 in 9. If you want to lob the heaviest rounds possible in the platform, 1 in 7.

Theoretically. Despite what "should" happen, individual barrels still show variations in the weights they "like".


January 11, 2004, 12:06 PM
Go to this site:


It tells you EVERYTHING to know about AR-15 ammo. Twists, differences, tolerances, velocity, etc...

A really informative site.

January 11, 2004, 12:16 PM
I just began reloading a couple of weeks ago.

My first reloading is for .223 for a single shot ruger.

Here is what I have learned about .223 twist.

With the original issue of the .223 barrels the twist was either 1 in 12 or 1 in 14. In fact the first M-16's were 1 in 12.

This is just adequate for up to 55 grain rounds (the original .223 bullet weight) However it is a bit marginal in a short barrel.

Bushmaster has a 1 in 9 twist. This if fast enough to stabilize all the way up to an 80 grain round even with a short barrel.

However, most fellows shooting 77 or 80 grain rounds are doing serious target shooting and opt for the 1 in 7 twist and a longer barrel.

My little Bushmaster shorty shoots 55 grainers with accuracy better than I can take advantage of with a 1 in 9 twist.

My Ruger does the same with 1 in 12 but I am shooting 40 grain hornady from it.

The lighter bullets (under 55 grain) stabilize just fine in a 1 in 12 barrel. Since my use is varmint shooting that is fine with me. Varmints are also the reason I opted for the shorty. Carrying in the woods is a piece of cake and the balance is excellent.

1 in 9 will serve you for about anything you want to do. Choose the barrel length based on what you want to do with the rifle.

January 11, 2004, 11:35 PM
So, the bottom line is, I shouldn't fire 55gr 5.56 ammo in a Rem. 700 PSS?


January 12, 2004, 01:36 PM
So, the bottom line is, I shouldn't fire 55gr 5.56 ammo in a Rem. 700 PSS?

If it's chambered for .223, the technical answer is 'no.'

It's basically a pressure difference. I've never heard of any issues from shooting 5.56 in a .223-chambered firearm, but I'm not about to be the first to have a story to tell either.

Though there's no explanation behind it, SAAMI's website (http://www.saami.org/unsafe3.htm) has some guidelines for unsafe arms and ammo combination. Notice it only warns against the 5.56 in a .223 chamber, not the other way around.

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