Use of 5.56mm ammo in .223 AR

[MOUNTAINORACLE]

This question came in our conversation and there was two different opinions.

I am in the camp that figures the the .223 and 5.56mm are the same; AR's chambered in .223 can fire the 5.56mm safely and 5.56mm chambered AR's can fire the .223 without problems other than accuracy due to the twist rates of the barrels.

The other camp claims the .223 can be fired safely in the 5.56mm chambered AR's but 5.56mm can not safely be in the .223 chambered AR's because of excess pressure.

I also have Bushmaster & Colt ARs that if memory serves me correctly their manuals claim you fire either without problem.

Thanks

oracle

[Captain Tuttle]

The other camp claims the .223 can be fired safely in the 5.56mm chambered AR's but 5.56mm can not safely be in the .223 chambered AR's because of excess pressure.

I believe this is correct. Especially for mil-spec ammo.

Both Bushmaster and Colt use mil-spec lowers which means they can handle both 5.56 and .223.

My two cents and worth half that.

[SteveXD]

.223 in a 5.56 is ok, but not the other way around...

[RocketFoot]

Agreed...my Bushy is .223 or 5.56, but some AR's may be .223 only.

[BerksCountyDave]

Yep. Look at the stamp on the barrel of the gun in question. If it's marked 5.56, you can use either. If it's marked .223, do not use 5.56. It is loaded to considerably higher pressures and using it in a .223 chambered gun can very well result in damage to the gun and/or the shooter.

[LittleRedToyota]

wikipedia cover 5.56 vs. .223 very well:

http://en.wikipedia.org/wiki/5.56x45....223_Remington

While the 5.56 mm and .223 cartridges are very similar, they are not identical. Military cases are generally made from thicker brass than commercial cases; this reduces the powder capacity (an important consideration for handloaders[1]), and the NATO specification allows a higher chamber pressure. Test barrels made for 5.56 mm NATO measure chamber pressure at a the case mouth, as opposed to the SAAMI location. This difference accounts for upwards of 20,000 psi (140 MPa) difference in pressure measurements. That means that advertised pressure of 58,000 psi (400 MPa) for 5.56 mm NATO, is around 78,000 psi (540 MPa) tested in .223 Rem test barrels. The 5.56 mm chambering, known as a NATO or mil-spec chambers, have a longer leade, which is the distance between the mouth of the cartridge and the point at which the rifling engages the bullet. The .223 chambering, known as SAAMI chamber, is allowed to have a shorter leade, and is only required to be proof tested to the lower SAAMI chamber pressure. To address these issues, various proprietary chambers exist, such as the Wylde chamber (Rock River Arms)[2] or the Armalite chamber, which are designed to handle both 5.56 mm and .223 equally well.

Using commercial .223 cartridges in a 5.56-chambered rifle should work reliably, but generally will not be as accurate as when fired from a .223-chambered gun due to the longer leade.[3] Using 5.56 mil-spec cartridges (such as the M855) in a .223-chambered rifle can lead to excessive wear and stress on the rifle and even be unsafe, and the SAAMI recommends against the practice.[4] Some commercial rifles marked as ".223 Remington" are in fact suited for 5.56 mm, such as many commercial AR-15 variants and the Ruger Mini-14, but the manufacturer should always be consulted to verify that this is acceptable before attempting it, and signs of excessive pressure (such as flattening or gas staining of the primers) should be looked for in the initial testing with 5.56 mm ammunition.[5]

[Kaos]

...
Both Bushmaster and Colt use mil-spec lowers which means they can handle both 5.56 and .223...

Just as a notation, the lower has nothing to do with the designation between 5.56 and .223.

[BerksCountyDave]

Just as a notation, the lower has nothing to do with the designation between 5.56 and .223.

Beat me to it. All in the upper.

[knighthawk06699]

Just as a notation, the lower has nothing to do with the designation between 5.56 and .223.

True cause you can run 6.8 safely out of a standard lower. All in the top end!!!

[Kaos]

True cause you can run 6.8 safely out of a standard lower. All in the top end!!!

There are even some bolt .50 uppers that use the standard AR lower too come to think of it.