5.56 and .223

[Colvin]

So my understanding has always been that one can fire a .223 round out of a 5.56 chambered AR-15, but you can't fire a 5.56 out of a .223 (at least, not a great idea). However, a seemingly knowledgable man told me that you can't fire .223 out of a 5.56 gun either.

Any merit to this? Thanks.

[SDF880]

I fire .223's out of my 5.56MM all the time for a long time and never any problem, just can't do that in reverse.

SDF880

[FrosSsT]

Yes you can fire .223 out of a 5.56 but like you said (because of pressure differences) you would be better off keeping 5.56 out of a .223

[kraigwy]

In 1978 I had the staff at the USAMU Sniper school help me set up a rifle for LE Sniping.

Rem. 700 BDL Varmint Special in 223, they recommend M193 so that's what I carried and shot in it until I retired in '94. Still had a bunch of Guard M193 and continued to shoot it until I ran out.

Of course we didn't have the Internet back then telling us we couldn't do it.

[coyota1]

I have fired 5.56 in my 788, two different 700's, and a ruger 77 with no pressure issues. No hard to extract, no primers popping out, no cratering. I guess pressures are higher, but I had no issues.

[Bart B.]

Interesting stuff on this in:

http://www.thegunzone.com/556v223.html

SAAMI's concerns are based on the chamber and cartridge dimensions, when there's the least difference in given cartridge and chamber sizes. Maximum spec 5.56 round in a minimum spec .223 chamber and bore can raise peak pressures higher than SAAMI feels is safe. That's based on the pressure issues SAAMI is concerned about.

So, as with all other product safety warnings and issues, one gets to decide their own margin of safety, then do whatever they feel is best for their objectives. It's my opinion that most folks reload belted "magnum" cases to a greater peak pressure over SAAMI specs than what 5.56 NATO rounds will have in .223 Remington chambers.

In comparing MIL SPEC pressure for the 5.56 M193 round to the SAAMI one for .223 Rem. SAAMI specs for the .223 max. average pressure are 52,000 CUP and 55,000 PSI. But the maximum average sample mean pressure for the .223 is 55,300 CUP, 58,300 PSI.

From MIL-C-9963F MILITARY SPECIFICATION: CARTRIDGE, 5.56MM, BALL, M193 (15 OCT 1976)Note "PSI" in MIL-C documents is actually CUP numbers; the military uses that term for copper crusher gauge units of pressure.
3.7.1 Measurement by copper-crush cylinder.-The average chamber pressure
of the sample cartridges, conditioned at 70° ± 2°F, shall not exceed
52,000 pounds per square inch (PSI). The average chamber pressure plus
three standard deviations of chamber pressure shall not exceed 58,000 PSI.
3.7.2 Measurement by piezoelectric transducer.-The average chamber
pressure of the sample cartridges, conditioned at 70° ± 2°F, shall not
exceed 55,000 PSI. The average chamber pressure plus three standard
deviations of chamber pressure shall not exceed 61,000 PSI.

[madcratebuilder]

The pressure difference (very little) is not the main concern. The 5.56 round with a heavy (long) bullet used in a .223 chamber with a short leade can theoretically jam the bullet in to the lands and grooves causing a pressure spike.

here's article I ran across that adds info.

5.56mm cartridges are loaded to a higher pressure (usually more powder) than a 223 Remington cartridge. This typically gives you more velocity with the 5.56mm round (barrel lengths being equal) than the 223 Remington. Both fire .224 caliber projectiles.

The difference between the 223 Remington and 5.56mm NATO chambers are in the throat area (also termed the "Leade" area). This is the portion of the barrel directly in front of the chamber where the rifling has been removed to allow space for the seated bullet. The throat in a 223 Remington chamber is typically .085" while the throat in the 5.56mm chamber is typically .162". The added space in the 5.56mm chamber compensates for the added pressure inherent in the 5.56mm NATO round.

The freebore area is the space that stands between the portion of the chamber machined to the fit the cartridge neck and the tapered beginning of the rifling in the barrel. This is also the distance that the bullet ogive (the point on the bullet that first contacts the rifling lands) has to travel before engaging the bore rifling.

The Leade is the full transition area from the end of the chamber to the full rifle land diameter. The beginning of the rifling is tapered to allow a gradual entry of the bullet to full rifling diameter (in our case, .224"). This encompasses the freebore area as well as the tapered rifling area.
Since the .223 Remington chambering dimensions are tighter than 5.56 NATO chambering, this can potentially create an over-pressure situation, exceeding SAAMI specifications, if you fire a 5.56mm NATO round in a 223 Remington chambered firearm. This overpressure situation is something that will surely damage your brass and potentially damage your firearms and you! This is the reason why you should not fire a 5.56mm NATO cartridge in a rifle chambered for 223 Remington.

The 5.56mm brass cases are a bit thicker in the walls and head than the 223 Rem. This is meant to add strength to the case for the higher pressures inherent in the 5.56mm round. While the added strength better contains the increased chamber pressure, this added thickness reduces the powder capacity and can further cause overpressure problems.

Since the 5.56 chambering is slightly more spacious, it is compatible with pressure profiles from either the 5.56 and/or .223. The larger chambering does not support the .223 cartridge as well (excess Leade dimensions leading to a larger bullet jump) and will cause a reduction in round to round consistency (which inevitably reduces round-to-round accuracy).

[Mobuck]

Out of at least 10 .223 chambered commercial rifles, I've found ONE that has a problem using M193 (type) 5.56 ammo. This is a NEF single shot that shows what I consider to be overpressure indicators when firing milsurp 5.56 55 grain FMJ. A couple of Savage 110's have run quite a bit of actual military (not commercial loaded to look like military) M193 ammo w/o any problems whatsoever.
I haven't/don't use the heavier bulleted 855 ammo in .223 rifles simply because I don't have any nor do I need to use that type of bullet for my purposes. There is the possibility of the longer bullet causing problems I haven't experienced.

[coyota1]

OK Bart, the pressure issue can be exacerbated by the shorter leede in the 223. Guns are proof tested for twice the pressure their pressure limit rating I believe. I really have no use for shooting 5.56 in a fine 223. It just seems pointless, and crude to do this in a fine well tuned 223 rifle

[603Country]

I'm surprised that nobody has suggested that if a fellow elects to use 5.56 ammo in his rifle, just check to see if that ammo is within his rifle's acceptable overall cartridge length. If it is, shoot it.

[jmr40]

People have been using them interchangeably since the inception. I've never known of a single problem.

Yes , there are 2 designations with very sligtly different specs. Many people never realise this, but you will see greater variations within the specs on both rifles and ammo for almost every chambering.

We only have designation for 30-06. But if you carefully measure the chamber dimensions on various rifles and measure chamber pressures on various ammo being sold you will see just as much or more variances as there is betweeen 223 and 5.56.

We could very easily break down 30-06 ammo and guns into 2 or 3 different classifications, but we don't. I've seen more problems with certain 30-06 ammo not working in certain guns than I have with shooing 5.56 in 223 guns.

[Bart B.]

coyota1 comments:

Guns are proof tested for twice the pressure their pressure limit rating I believe.

Twice is a bit much, I think.

Proof loads are typically 25 to 40 percent above normal average peak pressure. For example, the 5.56 M193 has a maximum CUP pressure average of 52,000 CUP. The 5.56 M197 proof load has a max standard pressure of 70,000 CUP; a 40% pressure over standard ammo.

By comparison, the 7.62 NATO and .30-06 round has 50,000 CUP max average pressure and their proof loads are 67,500 CUP max average; 35% higher for the proof load. Some 7.62 NATO handloads the military teams use in matches are at proof load levels for M1, M14 and AR10 service rifles. I've shot them and they do let you know they've got more gusto than standard service ammo.

[coyota1]

OK Bart, What I stated I believe applies to muzzle loading rifles, but not centerfire. I have worked up stout loads that showed signs of too much pressure, but I haven't had this with 5.56 in my 223's.

[madness]

not that i have gotten out my micrometer set to do this, but i do believe the .223 is the same diameter as the 5.56, both are 22's. the difference would be in the casing and load. i'm no gun expert by any means known to man or critter, but if it can be loaded onto the chamber it should be able to be sent down range. as for the pressure side of the eqaution, you really do need an expert to help you out there.

[allaroundhunter]

Guns are proof tested for twice the pressure their pressure limit rating I believe. I really have no use for shooting 5.56 in a fine 223. It just seems pointless, and crude to do this in a fine well tuned 223 rifle

Chambers and barrels are not engineered nor tested to have a factor of safety of 2 (max pressure/allowable pressure)**. That would use too much material, cost too much, and just be impractical to produce.

**most firearms, I am not going to say all as I am not sure about all firearms