Picked her up today, 10 days and 3 hrs after I paid for it. I had a grin on my face the entire 1 1/2 hr drive home. Put on the 1" riser w/ Truglo 30 mm red/green dot. Still have the bipod (came with 3 mounts) to install if I can figure out how to do it. Being an AR new guy I have a question. The barrel is stamped "556". I know the .223 and 556 are close in size but can I shoot .223 with this weapon? I also read that the 556 cannot be shot from a weapon stamped with .223. Ok, that's all the typing for me. Have to go back to the 15.

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Yes, with the 5.56 you can shoot .223.

Yes you can. Its recommended you dont shoot 5.56 in 223 bc the 5.56 generates more preassure and seats differently but since the M&Ps are chambered for 5.56 you can shoot either. There are some much better explanations online but hopefully I summarized it for you ok...

Congrats on the new rifle! I shot my new 15X for the first time today, nothing better!!!

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The Paper Assassin

congrats on finally getting her.

From the articles I have read the shell casing is thicker in the 5.56 rounds but have the same load as the .223 which accounts for the higher pressure in the chamber but no worries for you you can shoot either round. there are other differences but they are minute compared to the pressure one.

Plinker1962, congrats on the new M&P 15!

Thanks for the pictures. It's one of the few times I get to see a M&P 15 in its near O.E.M. state.

Barrels chambered in 5.56 can safely fire 5.56 NATO & .223 Remington.

Barrels chambered in .223 can safely fire .223 Remington. They can also fire 5.56, but it's not advisable.

Barrels chambered in .223 Wylde can safely fire 5.56 NATO & .223 Remington.

The key word is "safely". I know a .223 owner who fires 5.56 without issue, yet.


Yup. GMC Man, I searched for information on this a while ago. I tried to separate internet opinion from fact, and it started to give me a headache. These are the details that were consistent.

There is a difference in the chamber leade between a .223 and a 5.56 barrel that contributes to the pressure difference.

5.56 Chamber Leade > .223 Chamber Leade

A .223 round has less distance to travel before it contacts the barrel's rifling. This is part of the reason why .223 Remington is not as hot of a load as 5.56 NATO. The difference in chamber leade is also why a rifle chambered in .223 holds greater potential for accuracy than a rifle chambered in 5.56.

The tighter tolerance civilian .223 has a greater potential for being ammo sensitive. Not a big deal in a civilian rifle not intended for combat duty.

A 5.56 round has more distance to travel before it contacts the barrel's rifling. This is part of the reason 5.56 NATO is a hotter round than .223 Remington. The difference in chamber leade is why 5.56 holds less potential for accuracy than a rifle chambered in 5.56.

The minutely looser chamber tolerance 5.56 has less potential for being ammo sensitive. This is a good thing in a rifle intended for combat duty.

As for the cases being materially different: slightly longer, case wall thickness, etc, I've never been able to pin that one down.

I don't reload, but I try and pay attention to some reloading sub-forums. I generally see reloading advice that advises to reload .223/5.56 brass to .223 spec because:

1. Without a clear case head stamp, it's impossible to discern the difference between a .223 & 5.56 case. Better to be safe than sorry.

2. Even if they had verifiable 5.56 brass, I see complaints that some reloading manuals don't list 5.56 load data.

This is the point where I stopped researching the difference.

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-John

Congrats on your new "baby". I remember that long wait. I have a bad habit of buying guns around holidays, where I actually have to wait an extra day (Sport; Christmas '11 and Sig P226; Memorial Day '12).

Enjoy.

Thanks for everyone's posts. I don't have a need where I'm going to have to shoot 556 rds so it looks like I'll stick to .223's. Haven't taken her out yet, been trying to decide if I want a quad rail and I think I've talked myself out of it. Just the red dot and bipod will work for me. I have will power.

Congrats on a fine weapon. Now get out and shoot her!
BTW, it usually starts off with 'just this and that' then, slowly, imperceptably, our will power becomes practically non-existent.

That's what you say now.

To anyone reading this thread... If you've made a statement similar to this and you thought you had the willpower, raise your hand.

"All I need is <insert item 1> and <insert item 2> for my Sport. I don't think I'll need to spend any more money than that."

As for myself:

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-John

Yep I was just happy to have her, never thought I would want to change anything Yep that is what I thought. (slowly raises hand)
lmao

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Grover Oathkeeper

My hand is going up. I'm ordering a quad rail today. Dang I'm hooked!

and lo.....it begins!