AR 15 5.56 Ammo Question

[kleanbore 1]

Looking for some help from the knowledgeable folks here. I'm not an AR guy, only have the one and haven't fired it in years. I've read that 5.56 ammo should not be fired in a rifle chambered in 223. My rifle is marked on the lower, Colt AR-15 A2 Cal 223
Sporter II with serial # SP 310 xxx. Now the barrel in front of the front sight is lightly stamped C MP 5.56 Nato 1/7. The upper I believe is an A1 with the old style rear sight that you have to used a bullet point to adjust. Front sight takes a 4 prong tool to adjust elevation.I believe I read some where in the past that this is considered a transition rifle and was built this way by Colt and is all original. I'm assuming that since the barrel is marked 5.56 I can fire 5.56 even though the lower is marked Cal 223. Am I correct in this thinking. Like I said AR's are not my specialty. Now bolt actions, different story. Sorry for being long winded but I don't want to do anything dumb or stupid. All opinions appreciated. Thanks Brian

 

[mmcu]

As your barrel is marked 5.56 NATO, you're good to go with 5.56 ammo in your rifle.

 

[whatsupdoc]

You can shoot 5.56 or 223 in that rifle without issue.

 

[searcher]

I agree that you can shoot either 5.56 or .223 in that 1-&' barrel. If the barrel twist was slower than 1-7" AND had .223 on the barrel, I might be a bit hesitant, depending on the ammo used. With your rifle - I'd have no qualms shooting either.

 

[SecondsCount]

The difference between 5.56 and 223 is mostly the chamber. The throat is a little more generous in a 5.56 chamber.

5.56 ammunition is loaded to a higher pressure than 223, but externally, the case dimensions are identical.

 

[Herzo]

The 223 chamber is cut tight and can generate dangerous pressures with the 5.56 cartridge. A third (and common) chambering is the 223 Wylde. This is an accuracy chambering cut for the 5.56 and .223.
Barrel twist is irrelevant, the original M-16 was a 1:12 which would not stabilize a bullet heavier than a 55 grain. They later changed to 1:9 and more recently 1:7 to handle heavier bullets. SAMMI spec is substantially higher for the 5.56 than the .223.

 

[kleanbore 1]

Thanks everyone. I appreciate the information.