Was informed when I bought my new BB it is .223 but the PMAG states 5.56X45. I cannot find a designation on the rifle itself. It only lists as MODEL M&P 15 with no other designation. It is equipped with forward assist, dust guard, top rail with rear flip up sight A2 front sight and a serial# starting with 66XXX I have looked up and down the barrel under the hand guard but cannot seem to find the ammo designation. Any input as I want to take her out tomorrow and bought .223 rounds. THANKS FOR THE FORUM!

The only M&P 15 I know of in 223 is the 15I, sold outside the US. Assuming you are hear in the US and bought a M&P 15, it is chambered in 5.56. It can shoot either 223 or 5.56. The ammo you bought will be fine, enjoy.

Did ya look in your owners manual or on top of the barrel just behing the front sight?

5.56 vs .223 comes up as often as the sun. You will be fine. There MAY be some pressure problems when using certain surplus military 5.56 NATO ammo in SOME .223 Remington chambered rifles. Has to do with the bullet ogive hitting the rifling leade in the throat area. But most 55-62gr .223 commercial ammo is perfectly safe in your rifle.

Ruger marks their Mini-14 barrels .223 Rem , yet the manual states it's fine for 5.56 military ammo.

Dragon THANKS! First AR and just wanted to verify since momma is shooting. I thought that was the case but just wanted to make sure!

This PDF will help your understanding of the two rounds.

http://le.atk.com/pdf/223VS556.pdf

Try to find your rifle here.................. http://www.smith-wesson.com/webapp/w...51_757784_-1_Y

No problem, enjoy the gun. Since it is your first AR, I would suggest reading up on and browsing Youtube for info on cleaning, maintenance, accessories, etc. Tons of good info here on the forum too.

I did my old blindfold marine breakdown and funny I STILL HAVE IT! Momma was laughing the whole time! Thanks guys I am sure happy this forum is out here and will post pics on its build up!

Just found it! M&P15-MOE! Found in the website MPDC sent and found designation paperwork under the packing foam in the hard case. Getting more and more excited!

I have read that you CAN fire .223 out of a gun chambered for 5.56 but you are not supposed to shoot 5.56 out of a gun chambered for .223.

It has to do with the leade of the barrel. Ill post this from another forum:
* The 5.56mm and .223 Rem chambers are nearly identical. The difference is in the "Leade". Leade is defined as the portion of the barrel directly in front of the chamber where the rifling has been conically removed to allow room for the seated bullet. It is also more commonly known as the throat. Leade in a .223 Rem chamber is usually .085". In a 5.56mm chamber the leade is typically .162", or almost twice as much as in the 223 Rem chamber.
* You can fire .223 Rem cartridges in 5.56mm chambers with this longer leade, but you will generally have a slight loss in accuracy and velocity over firing the .223 round in the chamber with the shorter leade it was designed for.
* Problems may occur when firing the higher-pressure 5.56mm cartridge in a .223 chamber with its much shorter leade. It is generally known that shortening the leade can dramatically increase chamber pressure. In some cases, this higher pressure could result in primer pocket gas leaks, blown cartridge case heads and gun functioning issues.
* The 5.56mm military cartridge fired in a .223 Rem chamber is considered by SAAMI (Small Arm and Ammunition Manufacturers Institute) to be an unsafe ammunition combination.

Also 5.56 is loaded to a higher pressure than .223, that plus the tighter .223 chamber can cause problems. Not likely to blow up the gun but can cause popped primers and extraction problems.

Congrats and welcome to the forum! I'd be pretty dang excited too if I were looking at a new M&P15 MOE. It's a mean, clean, great looking rifle IMO.

Many knowledgeable folks on this forum. I've learned a lot here. Looking forward to some photos and a range report.

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JT