Set me straight on my rifles (.223 and 5.56)

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copeland

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So I know that it is recommended you only fire .223 and 5.56 in a 5.56 rifle.

However, here's where I need some clarification on my rifles, and this is mainly to curb my curiosity.

1.) Bushmaster XM-15 E2S - 16 inch Heavy barrel.

- Lower stamped 5.56/.223 so I assume it'll handle both. Website confirms this. However, the barrel is stamped "B MP .223 REM 1/9".

Does this affect ammo choice as far as .223 vs 5.56? I'm just wondering why it would have a .223 barrel

2.) Stag Arms - 14.5 inch heavy barrel

-Lower stamped .223. But the upper is a 14.5 Bushmaster stamped "B MP 5.56 NATO 1/9".

Does the .223 lower affect anything in relation to the chamber when firing 5.56?

3.) RRA lower - New.

If the Stag lower poses some concern for firing 5.56, maybe I should swap it's 5.56 upper to this RRA lower?

I apologize if these questions have been beat to death. I'm just curious as I'm not finding the answers online...

Thanks.
 
The barrel marking is what determines the caliber you can use, it not only specifiy the bore of the barrel, but more importantly what chambering it is.

Unless the lower is for ammo that uses different casings thant is not in the .223/5.56 family, the markings on a lower it is more a beaurcratic excerise than anything else.
 
There are also rifles marked .223 specifically for export to the Canadian market such as the H&K MR223, but can be used with 5.56 ammo.
The U.S. as well will not export any rifle stamped 5.56, and to get around export restrictions they will stamp 5.56 barrels with .223.
The best thing to do is contact the rifle manufacturer to see if this is the case.
This is only the case with a very select few rifle makers!!! Do not assume it is safe to shoot any rifle stamped .223 with 5.56 ammo!!!
 
Are .223 and 5.56 not the same caliber?

I thought the only difference was chamber pressures since 5.56 is a slightly hotter load. I was under the impression that the only difference is in the upper receiver, as to whether or not a .223 receiver can handle the pressures of a 5.56 load
 
Onagoth said:
Are .223 and 5.56 not the same caliber?

I thought the only difference was chamber pressures since 5.56 is a slightly hotter load. I was under the impression that the only difference is in the upper receiver, as to whether or not a .223 receiver can handle the pressures of a 5.56 load
Click to expand...

i was going to type out a long explanation, but i found another one that says it better than me:

1) The chamber dimensions between the two cartridges are different. 5.56mm has a longer throat or leade (the the unrifled portion of the barrel between the end of the chamber and the beginning of the rifling). This longer throat reduces pressures as compared to the .223 Remington. Consequently, .223 Remington ammunition is loaded to a lower pressure.

2) The 5.56mm cartridge is loaded to higher pressures than the .223 Remington. Taking advantage of the longer throat, the 5.56mm can safely operate at a more powerful loading level.

Firing a 5.56mm cartridge (which is loaded to higher pressure) in a .223 Remington chambered rifle (which has a shorter throat causing a further boost in pressure), could exceed the safety margin in a .223 Remington rifle. It cannot be done with complete safety.

A .223 Remington cartridge can be safely fired in a 5.56mm chambered rifle, but NOT vice versa

the difference is in the barrel's chamber
 
Why they bother stamping cal. on the lower is beyond me...

You can have a lower marked .556, but once mated to an upper it can be anything from .22lr to .50BMG
 
Clubhopper said:
i was going to type out a long explanation, but i found another one that says it better than me:

1) The chamber dimensions between the two cartridges are different. 5.56mm has a longer throat or leade (the the unrifled portion of the barrel between the end of the chamber and the beginning of the rifling). This longer throat reduces pressures as compared to the .223 Remington. Consequently, .223 Remington ammunition is loaded to a lower pressure.

2) The 5.56mm cartridge is loaded to higher pressures than the .223 Remington. Taking advantage of the longer throat, the 5.56mm can safely operate at a more powerful loading level.

Firing a 5.56mm cartridge (which is loaded to higher pressure) in a .223 Remington chambered rifle (which has a shorter throat causing a further boost in pressure), could exceed the safety margin in a .223 Remington rifle. It cannot be done with complete safety.

A .223 Remington cartridge can be safely fired in a 5.56mm chambered rifle, but NOT vice versa

the difference is in the barrel's chamber
Click to expand...

Pretty much how I see it. Although I don't think it poses any "ka-boom" risks. The only slightly possible risk of shooting "5.56 nato" loaded rounds in a .223 chamber is wearing up the throat faster. But no data that I know of prove or disprove my theory.

As some might say: Before the internet came along it was safe to shoot either in either chambers.

Marking on the lowers doesn't say anything about the caliber of the upper. Some lowers are even marked "multi" because you can build up an AR in so many ways.
 
alexD92 said:
Pretty much how I see it. Although I don't think it poses any "ka-boom" risks. The only slightly possible risk of shooting "5.56 nato" loaded rounds in a .223 chamber is wearing up the throat faster. But no data that I know of prove or disprove my theory.

As some might say: Before the internet came along it was safe to shoot either in either chambers.

Marking on the lowers doesn't say anything about the caliber of the upper. Some lowers are even marked "multi" because you can build up an AR in so many ways.
Click to expand...

yeah i agree. i don't think it'll pose a risk of blowing up in your face, but just cause more wear.

just in case, i'd definitely want to inspect the spent brass and check signs of overpressure
 
I was suspect that the lower had nothing to do with. But I'd figured I'd better make it crystal clear.

So as far as my two uppers then, it seems I'm good to go as my bushmasters are either 5.56 (14.5) and both (16")

I know the topic has been beaten to death but this thread has taught me about the leade - I'll read up more on this.
 
Just to throw one more twist into this debate, the pressure specs for ammunition vary by the organization that sets the standards. Just for fun, look up the CIP max pressures for both cartridges, and then compare them to SAAMI specs.
 
5.56 mm (5.56x45mm NATO)

Service Pressure Pmax
MPa / (psi) 430.0 / (62,366)

Proof Round Pressure Requirement
MPa / (psi) 537.5 / (77,958)


Wikipedia
According to the official Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (C.I.P.) guidelines the .223 Remington case can handle up to 430 megapascals (62,366 psi) piezo pressure.

The SAAMI pressure limit for the .223 Remington is set at 379.212 megapascals (55,000 psi), piezo pressure

So 223 is reaosnably weaker...however : In C.I.P. regulated countries every rifle cartridge combo has to be proofed at 125% of this maximum C.I.P. pressure to certify for sale to consumers.
 
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