Actually, I was wrong. My M&P was marked .223....I figured they all were. But I just looked it up on the SW site and the ones the cops use is the M&P Sport, which is listed at 5.56. (I just went down to check mine out and it says .223. But mine is more than 10 years old.)
https://www.smith-wesson.com/product/m-p-15-sport-ii
I wonder when THAT changed.
Thanks for checking. The AR world is full of craziness; I shouldn’t have been quite so sure of how they’re marked.
Personally, I’ve only ever seen “5.56mm” and “.223 Wylde”.