Hi guys. I bought an MP-15 a few months ago and wanted to finally take it to the range this weekend to do some plinking. I read where it said you should not use 5.56mm in a weapon chambered for .223 in the instructions. That having been said, I consulted my cardboard box that it came in and it says 5.56 on the exterior sticker, but not NATO. My MP-15 does not have any marking on the exterior of the gun to indicate what is the appropriate caliber. Now I am scared and do not know what I should fire through this as the instruction manual is very clear on this. I know I will be ok either way with .223, but why would I have a cardboard box marked 5.56? Sorry for the probably novice question but I wanted to be sure before I take my wife and children with me to the range. Any input is as always appreciated. Thank you.

5.56 is the NATO designation for the Winchester .223 round. Technically the full NATO designation for the 5.56 is 5.56X45mm, but most of the time (all the boxes on the ship) just have the 5.56 cartridge designation. The 5.56 designation is what you need to be able to shoot eithe rthe 5.56 or the .223 ammo out of it.

Similarly the .308 Winchester is also know as NATO 7.62X51MM.

So yes, you can shoot .223 ammo in your rifle.

Hope this helps.

Both of my M&P's have 5.56 NATO 1/9 stamped on the barrel near the muzzle.

If you have not done so and I suspect you haven't, clean the rifle inside and out and lube it before taking to the range. Check Youtube for videos on how to do so.

Yes sir, appreciate your response. I guess what I am trying to ask is can I shoot 5.56mm out of this weapon? I have read that the pressures are too extreme and I could have problems. The box says 5.56 but as it doesn't say NATO or the mm addition. I am leery or shooting anything but .223 through it. I guess I just wonder why you would mark a box 5.56 when all I may be able to use us .223. Thank you nonetheless for helping me.

Thanks MichiganScott. Your are correct. I have the same markings on my muzzle. Am I ok to shoot the NATO 5.56 round through this weapon with no fear?

Shoot 5.56 without fear. Enjoy.

Thank you sir, most appreciated

It's the exact same round, just written out more completely in different places. If it has "5.56" listed anywhere, it's referring to the same ammo. 5.56=5.56mm=5.56 NATO=5.56x45mm

There aren't variations of 5.56mm ammo. With things like .22, there is .22lr, .22 long, .22 short, etc. And .17, there is .17hmr, .17hm2, .17WSM, etc. Not so with 5.56 there is only one round with that designation (just written different ways sometimes).

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686/M&P9 Pro/M&P15X/M&P15-22

I learned the 5.56 chamber is different than 223. not only for pressures but in length and in how far the round is into the rifling when chambered- if in doubt email S&W with you sn# to be sure

Just know that .223 Winchester and 5.56 NATO are NOT the same thing. Sure it will chamber, but DO NOT use 5.56 NATO rounds in a rifle only marked for .223 Winchester. The reverse (using .223Winchester in a 5.56NATO capable rifle) is fine however.

Lots of info speaking to this online.

Ask Foghorn: What's the Difference Between 5.56 and .223? | The Truth About Guns

Yes, .223 Remmington and 5.56 NATO are different rounds. .223 can be used in firearms marked for 5.56, but 5.56 should not be used in firearms chambered only for .223

There are no Smith and Wesson M&P15's chambered in .223 only. They are mostly 5.56 NATO with a few in 300 BLK.

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686/M&P9 Pro/M&P15X/M&P15-22

Most AR's nowadays will be 5.56. Though some companies like JP Rifles chamber theirs in .223 wylde. Not to be confused with .223 Rem. In a .223 wylde rifle you CAN shoot 5.56NATO.

In fact; any round you shoot in a 223 Wylde will work, but they all come out as a 223 Wylde!
Ackerman 223 also.

Just as there are pressure differences between 5.56 and .223, there are differences between .308 and 7.62x51, with .308 having higher pressures.

This is incorrect. There are some .223 M&P 15s. They are produced for international markets. You would have to look to find one in the US, but yes, they do exist.

That's really weird, as the .223 is a U.S. civilian round, we are the only country that uses inches -the designation of the round. 5.56mm is the universal round, found the world over. Strange that the M&P would be found in that .223 designation only outside of the U.S.

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686/M&P9 Pro/M&P15X/M&P15-22

In some countries, civilians are not allowed to own military and/or law enforcement calibers.

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-John

Vinnie, since you seem to doubt me...

Product: ARCHIVE: Model M&P15I

Would a .223 chamber be due to wanting the shorter head space between the .223 and 5.56? I know a tighter chamber in the 10-22 bull barrels usually equates to greater accuracy since the bullet engages the rifling sooner.

ASTMedic, If all you were going to fire were .223, you might be better off with the .223 chamber. Since the jump to the rifling is longer in 5.56, it does seem logical that .223 out of a .223 chamber would be more accurate.

With that said though, I'm not sure what the real world gain would be. I've seen guys on this board handload rounds that stacked right on top of each other at 100 yards.