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What is the difference between 223 & 556


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agreed. I was a total newb to AR's and was told to buy one chambered in 5.56 as opposed to 223. That way any ammo I buy would be good to go. I think there are some slight differences elsewhere but pretty much as PB said. 5.56 is hotter.

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.556 is a bit looser on the headspace to facilitate feeding. 223 chamber has a tighter tolerance on the headspace. If you shoot 556 in a 223, you can have pressure issues . It supposedly makes a 223 a bit more accurate with factory ammo. You can set headspace, if you reload, to match your gun. There are a number of threads on this subject on the A/R-15 forum. Or so I heard. I never go to any other forums. :devil:

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Load away. Same dies. Shoot em' up. My colts are .556. I load with .223 dies. I use LC brass with 24 gr H-4895 and 69 gr siera BTHP and 2.25 OAL. Shoots 2900 fps and a 1.5-2 inch group at 100 yds with iron sights. YMMV.

Edited by Youngeyes
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There are differences. First, the 5.56 is a military round, generally with a velocity in excess of 3100 fps. It has a higher pressure than a .223. The .223 was originally designed for bolt guns. You can shoot heavier bullets in .223 bolt gun that are longer and generally will not fit in a magazine gun magazine. Where the bullet meets the lands are usually different between the calibers. Military brass is usually thicker to handle the pressures, the primers are different and the primer pocket usually needs swaging.

There is also a Wylde chamber which is a compromise between the 5.56 and .223.

There is a lot more information available on the internet, the above is just the high points.

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As others have said, it's primarily pressures that the ammunition is loaded to and the amount of freebore in the chamber.

SAAMI had the drawings and chamber specs on their website. Freely available to download here: http://saami.org/specifications_and_information/index.cfm

Info sheets from Federal and Armalite are attached for your consideration.

Is there a specific reason concern you have that we can help with?

223VS556.pdfTech Notes_TECH NOTE 74 5.56 vs 223 vs Wylde 090817 Rev 0.pdf

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The first article says in the last paragraph that some commercial barrels are marked 556 but are actually cut for 223. So if this is true you shouldn't fire a 556 in a barrel that's not cut right. How do you measure the Leade in the chamber?

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The first article says in the last paragraph that some commercial barrels are marked 556 but are actually cut for 223. So if this is true you shouldn't fire a 556 in a barrel that's not cut right. How do you measure the Leade in the chamber?

I measure by casting the chamber with cerrosafe. When I do a chamber cast I usually cast the first 2 to 3 inches of the barrel so I can measure land & groove dimensions, etc. It's easy to do and fairly inexpensive. Cerrosafe is an alloy that melts around 200 degrees fahrenheit. Follow the instructions that come with it, and it's relatively foolproof. Compare the dimensions to SAAMI and NATO specs.

You can also do this with lead, but that's a messier and more difficult process. It is a bit more accurate tho.

-john

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The first article says in the last paragraph that some commercial barrels are marked 556 but are actually cut for 223. So if this is true you shouldn't fire a 556 in a barrel that's not cut right. How do you measure the Leade in the chamber?

I wouldn't worry too very much about this tho. I've never encountered a mismarked AR barrel. I handload 223 with Varget and run it pretty close to NATO pressures with some bullets. I've never had a problem doing this, but I also load to magazine length so the bullets are nowhere near the end of the leade when chambered.

What brand barrel are you concerned about?

-john

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The case volume of a military case can be lower due to thicker walls, this causes higher pressures which can be dangerous if you load near the maximum loads.

Measure case capacity (with water for example) and see for yourself.

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  • 2 weeks later...

i think it has to do with the neck dimensions and the SAAMI pressures. If you want ultra accuracy - you would want the chamber to match the cartridge I suppose...

Yeah 556 is 223 +P. I have seen lots of 556 LC brass with out of spec necks.

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My ar's are chambered for 556 but I load with 223 dies. My brass is lake city and I have it ready to load a bunch of rounds.

556 and 223 are the same cartridge. The difference is pressure. You will find quite a large variation in 556/223 cases by manufacturer. 556 cases really do not have less case capacity than cases marked 223. On average 556 marked cases have a tiny bit more case capacity than the majority of 223 cases.

Some 556 marked cartridges have out of spec necks because they can get away with it. Just trim to spec and work up your loads by starting in the middle and moving up. There is both 223 pressure and 556 pressure load data available.

Edited by mboylan
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  • 1 year later...

"The case volume of a military case can be lower due to thicker walls, this causes higher pressures which can be dangerous if you load near the maximum loads.

Measure case capacity (with water for example) and see for yourself."

Not true. Most LC cases have the same or more capacity then commercial cases when talking 5.56. 7.62 cases are another story as they do have less capacity. As always work up etc. There is NOT a practical difference between .223 and 5.56 in practical terms other then the increased PSI spec for 5.56 vs .223. Some .223 reamers are bigger then 5.56 and some 5.56 are smaller then .223. Free bore in the throat area is another practical matter.

Since most of us dont have a pressure guage at home i might suggest that many shooters are loading hot ".223" ammo that is in excess of 5.56 pressures especially when you are talking 80-90g bullets.

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I never thought shooting LC M855 factory ammo out of my Savage model 12 heavy barrel .223 would be a problem. Still don't think it is unless you guys tell be different. (only reason I have done it is because it's all I had at the time and haven't gotten around to loading for it yet)

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The only difference that matters between the .223 and 5.56 is the throat, "BOTH" cartridges are loaded to 55,000 psi. The difference happens when you shoot a 5.56 military cartridge in a short throated chamber because it raises the chamber pressure. I have two AR15 rifles and a bolt action .223 Savage and its throat is longer than either of my ar15 rifles. The short .223 throat below is .0250 in length, the longer 5.56 throat is .0500 and the throat in my Savage .223 is .0566. And what many firearms manufactures are doing now is making the chamber big enough to fit either cartridge "PLUS" room for the company lawyer to fit in also.

The SAAMI interchangeability warning for the .223 and 5.56 came out in 1979 when the military changed to the M885 cartridge and lengthened the throat in the M16 rifle. Bottom line if the rifle has a 1 in 14 or 1 in 12 twist chances are it has a short throat and designed for lighter bullets. My Savage .223 has a 1 in 9 twist and a throat longer than a AR15 rifle and also do not confuse European CIP pressure taking methods that reads 5,000 psi higher than American SAAMI methods.

223_zps6248614d.jpg

Below look at the various throat diameters and throat lengths and remember SAAMI chamber dimensions are guidelines and do not have to be followed by gun manufactures.

223-556_zpsf4f56449.jpg

Also Lake City cases are "NOT" thicker and have the most case capacity, BUT Lake City cases are made of harder brass in the base and a thicker flash hole web to better withstand larger diameter military chambers.

casecap_zps3f8bb2c9.jpg

hardness-a_zps8d54ad66.jpg

556hard-a_zps7570e6b0.jpg

Below is the milspec requirment for comertial contract 5.56 ammunition made for the military, please notice the chamber pressure is 55,000 psi the same exact pressure the .223 is rated for.

m855specs_zps15389a22.jpg

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