it depends on the chamber:
http://www.ammo-oracle.com
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Fact: SAAMI specifically warns against the use of 5.56mm ammo in .223 chambers._ The .223 SAAMI specification was originally made with bolt rifles in mind.
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 SAAMI .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.
5.56 v. .223 Remington specification.
See also: Remington's description of the differences between .223 and 5.56mm.
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.
The FAQ section of
www.fultonarmory.com has some .308 data
http://www.fulton-armory.com/308.htm
What's the Difference between .308 Winchester & 7.62x51mm NATO?
by Clint McKee and Walt Kuleck
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dumb question i alway thought these 2 ammos where interchangeable but some have told me otherwise whats the story??? jim
Hi, Jim,
This is a perennial topic, kinda like ".45 vs. 9mm" or "Best Guns & Loads for Deer."
They are not the same.
They are the same.
They are not the same, 'cause the .308 Win was released by Winchester several years before the Army standarized the T64E3 as the 7.62MM. You'll get an endless discussion of pressure specs, endless because SAAMI and the Ordnance Dep't measured pressure in different, unrelateable ways. Howver, the chamber drawings are different.
They are the same, 'cause nobody (and Clint's been looking for many years!) makes 7.62MM ammo that isn't to the .308 "headspace" dimension spec. So 7.62MM ammo fits nicely into .308 chambers, as a rule.
But in some 7.62MM rifles the chambers are long (to the 7.62MM military spec), notably the Navy Garands with 7.62MM barrels. Thus, using commercial ammo in such a rifle is not a good idea; you need stronger brass. Use military ammo or the best commercial only, e.g., Federal Gold Medal Match.
Most of the time it's a distinction without a difference. But if you intend to shoot .308 commercial in a military arm chambered for 7.62MM, first check the headspace with .308 commercial gauges first. You may get a surprise.
Best regards,
Walt Kuleck
Fulton Armory webmaster
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Clint, What's the difference between .308 Winchester & 7.62x51mm NATO?
Jerry Kuhnhausen, in his classic Shop Manual (available from Fulton Armory; see the M1 Rifle Parts & Accessories or M14 Rifle Parts and Accessories Pages under Books) has published a somewhat controversial recommendation concerning .308 Winchester and 7.62x51mm NATO ammo, headspace & chambers. I broached the subject with him some months ago. He had his plate full, so we decided to chat on this in the future. When we do I'll report the results of our conversation.
I completely agree with Jerry that if you have a chamber with headspace much in excess of 1.636 (say, 1.638, SAAMI field reject), you must use only U.S. or NATO Mil Spec Ammo (always marked 7.62mm & with a cross enclosed by a circle) since the NATO mil spec calls for a far more "robust" brass case than often found in commercial (read .308 Winchester) cartridges. It is precisely why Lake City brass is so highly sought. Lake City brass is Nato spec and reloadable (most NATO is not reloadable, rather it is Berdan primed). Indeed, cheaper commercial ammo can fail at the 1.638 headspace (e.g., UMC) in an M14/M1 Garand. Many military gas guns (e.g., M14 Rifles & M60 Machine guns) run wildly long headspace by commercial (SAAMI) standards (U.S. Military field reject limit for the M60 & M14 is 1.6455, nearly 16 thousandths beyond commercial (SAAMI) GO, & nearly 8 thousandths beyond commercial (SAAMI) field reject limit!).
I also agree that 1.631-1.632 is a near perfect headspace for an M14/M1A or M1 Garand chambered in .308 Winchester. But I think that it also near perfect for 7.62mm NATO!
I have measured many, many types/manufacturers of commercial and NATO ammo via cartridge "headspace" gauges as well as "in rifle" checks. If anything, I have found various Nato ammo to be in much tighter headspace/chamber compliance than commercial ammo. Indeed, sometimes commercial ammo can not be chambered "by hand" in an M14/M1A with, say, 1.631 headspace (bolt will not close completely by gentle hand manipulation on a stripped bolt, although it will close & function when chambered by the force of the rifle's loading inertia), though I have never seen this with NATO spec ammo. I.e., if anything, NATO ammo seems to hold at the minimum SAAMI cartridge headspace of 1.629-1.630, better than some commercial ammo!
So, why set a very long 1.636 headspace in an M14/M1A or M1 Garand? It probably is the conflict mentioned above. Military headspace gauges say one thing, SAAMI headspace gauges say something else, as do the spec's/compliance covering ammo. In a court of law, who will prevail? I think Kuhnhausen gave all those who do this work a safe way out. However, I believe it not in your, or your rifle's, best interest. Whether you have a NATO chambered barrel (M14/M1 Garand G.I. ".308 Win."/7.62mm NATO barrels all have NATO chambers), or a .308 Winchester chamber, keep the headspace within SAAMI limits (1.630 GO, 1.634 NO GO, 1.638 FIELD REJECT). This subject is a bit confusing, and for me difficult to explain in a one way conversation!
Clint McKee