I’m new to the AR platform and have a Sport II. I have been using American Eagle .223. When looking at 5.56 I see several different designations: 5.56x45, 5.56 NATO, etc.
What is the correct 5.56 for this rifle?
Thank you.

Those are one and the same.

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Or something like that . . .

Although .223 Remimgton and 5.56 NATO (5.56x45) are pretty much the same, (The .223 Rem is basically the 'civilian version' of the 5.56 NATO, here is what is important to know about the difference between the 2 cartridges;


The differences between the two are small but can have a large impact on performance, safety and weapon function.

The first difference is the higher pressure level of the 5.56 NATO cartridge which runs at approximately 58,000 psi. A 223 Remington is loaded to approximately 55,000 psi.

The second and most important difference between the two is the fact that a 5.56 NATO chamber has a .125” longer throat. This allows approximately one more grain of powder to be loaded into a 5.56 NATO cartridge; this is what gives it higher performance than the .223 Remington.

The biggest problem with these differences is when firing a 5.56 NATO cartridge in a rifle chambered for .223 Rem. Due to the longer throat that the NATO chamber employs this combination will cause a .223 chambered weapon to run at approximately 65,000 psi or more. This is 10,000 psi higher than the .223’s normal functioning pressure of 55,000 psi. This is NOT safe and will cause primers to back out, or worse, cause harm to the operator, the rifle, or both.

The reverse of this is firing a .223 Rem cartridge in a 5.56 NATO chambered rifle. Due to the throat difference between the two chambers a 223 Rem cartridge may not work optimally in a 5.56 NATO chambered weapon. The cause of this is the lack of pressure built by a 223 Rem cartridge fired from a 5.56 NATO chamber. The 223’s 55,000 psi will not be attained and therefore velocity and performance are hurt. Problems start occurring when this combination is fired out of a 5.56 NATO chambered rifle with a 14.5” (or shorter) barrel. The lower powder charge of the .223 round coupled with the pressure drop that occurs when it is fired in a the 5.56 NATO chamber will cause the rifle to cycle improperly. NATO chambered rifles with barrels longer than 14.5” should function properly when firing .223 Rem ammunition.

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SIC SEMPER TYRANNIS

Eric, wonderful explanation for more experienced shooters , I just hope it doesn't confuse the OP!

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Old paratrooper in NC

Gotta learn it sometime, and, there's no time like the present!

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SIC SEMPER TYRANNIS

Many of the newer AR 5.56mm caliber rifles now have the "Wylde chamber." This is designed to handel both 5.56mm and .223 Remington rounds interchangeably.

I don't know what chamber the Sport II uses, but it might illuminating to check which of the three chamber types that it has.

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Keep your powder dry.

The Ruger Mini14 also can fire both safely.

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S&W BG380, S&W 915 9mm

The Spirt II has both the 5.56 and.223 designation.
Eric, great explanation! That said, if 5.56x45 @nd 5.56 NATO are the same, are there any 5.56 designations,grains, etc to stay away from for the Sport II.
Thanks

The 1/9 twist barrel may have difficulty stabilizing heavier (longer) bullets - say those over 72 grains. Thus it may be best to avoid those, but some people claim they are accurate from their 1/9 barrels. It's probably only an issue if you are shooting at longer ranges than most people do with an AR, though.

Mine has shot everything I've fed it from 50gr. up to 68. I shoot mostly 55 and 62 grain .223 ammo, though, because it's generally less expensive.

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Or something like that . . .

1/9 is a good all around twist rate. All of my .223 and 5.56 rifles have 1/9 on purpose. My bolt rifles like 55gr. My 5.56/.223 likes 62 gr. I wish they all had .223 chambers but few semi autos do.

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That's just somebody talkin.

I'm an experienced shooter and still scratch my head on this one...

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NRA Life Member

Any .223 or 5.56 will do, don't make this thread a information overload.