So, here is the advertisement for the S&W M&P15 Sport2.
SKU 10202
Model M&P®15 Sport™ II
Caliber 5.56mm NATO/.223
Capacity 30
I have been in email conversations with S&W trying to get them to answer the question, why do they advertise both calibers. So far this is what they told me in response, "Being that the 5.56 NATO has a higher pressure than the .223 The rifling/barrel settings can accommodate both rounds effectively.
Does anyone know what they're talking about? RIFLING/BARRES SETTINGS? What the heck is that?
I didn't know there were settings on the rifling and barrel. There are chamberings but as far as I know there is no way to chameber for both calibers.

I believe the proper term is “specifications”. If it is marked 5.56/.223 it will safely fire and function with both ammo types. There is also a .223 Wylde chamber; a slightly different chambering to accommodate both 5.56/.223 ammo.

It's not just S&W that stamp 5.56/.223 on their lowers. Most all AR manufacturers do it as well. Some also will stamp Multi, (and Wylde as mention in post #2). All meaning the rifle will chamber/shoot both 5.56, and .223 cartridges. There are minor differences between the two. Why gun manufacturers continue to stamp that info on their lowers I will never know, since the lower really has very little to do with it.

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SIC SEMPER TYRANNIS

I was under the impression that if it's stamped 5.56 it can soot both but if it's stamped .223 it can only shoot .223. I don't have any issues firing either through my sport II so I don't lose sleep over it.

Honestly, folks make way too much of an issue out of the whole .223/5.56 chamber thing. I'd be more concerned with shooting unknown (5.56/.223) reloads out of an AR before I was even a little concerned about shooting factory 5.56 through a .223 chamber. I seriously doubt an AR is going to Ka-Boom due simply to the fact that a 5.56 round was fired through a .223 chamber. Actually, I have never seen it happen, heard it happen, or read about it happening. But that's just me....

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SIC SEMPER TYRANNIS

Man, you leave a teapot by itself for a few minutes, and the next thing you know there's a tempest in it.

The Internet wizards have taught me how little I know about guns and ammunition. I am truly amazed at how gun owners have managed to survive all of those years before Al Gore invented the Internet.

My personal experience is there isn’t enough difference between the .223 and 5.56 to even think about. I know shooters that have and still use it without any mishaps. If shooting the 5.56 in .223 chambered guns are so dangerous where are the photos and posts documenting and showing the damage?

Their response is self-explanatory and spot-on. Basic info if you plan on participating in the AR / firearm arena. At this time I'd advise against purchasing a 300blk.

I often wonder why after all these years, why do we still need both? Choose one and go with it.

Since no one has yet answered the question specifically... :

The two rounds are the same in size. 'In theory', a .223 Remington chamber is cut to smaller dimensions than 5.56 NATO, but practically in almost all instances this makes no difference in use.

A very tight .223 chamber using an overpressure 5.56 mm round 'may' cause problems but this is unlikely, and there is so much production variation between companies that a loose 5.56 chamber will be within .223 specs.

Most manufacturers have gotten around the whole issue by making 'all' their chambers 5.56 spec. They just stamp these are suitable for .223 to avoid discussions exactly like this one.

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Alan
SWCA LM 2023, SWHF 220

Other members have explained why S&W advertises both calibers on the M&P15 Sport 2, but many seem to be taking the advisory lightly.

This is from SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute):



Faqs – SAAMI

Others may get away with it, but I’ll continue to heed the warning. I only have 2 rifles with 5.56 chambers and nothing with a .223 Rem chamber, so it’s a moot point with my guns.

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"I also cook."

Simple answer...

5.56mm and .223 are very nearly identical in case sizing. A firearm chambered for 5.56mm can fire both. If your chamber is marked .223, you should not fire 5.56mm rounds in that firearm. It may not be rated for the higher pressures.

To my knowledge, the only M&P 15s made with a .223 marked chamber were produced for the international markets, for countries where the citizens cannot have military cartridges.

Take a few minutes and read this. Very educational regarding 5.56 v .223.

https://www.luckygunner.com/labs/5-56-vs-223/

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SIC SEMPER TYRANNIS

So, to answer your question , " Why does S&W advertise 5.56 and .223" The simple answer is because it can fire both 5.56 and .223.