Hi all,
I was reading that there is actually a difference in the way these two cartridges sit in the chamber. I read about "leade" and how if a 556 is chambered in a 223 weapon, the leade isn't enough and there can be excess back-pressure. So maybe practically things are the same, but what is the "leade" on the M&P? .085 or .162?
.223 Remington vs. 5.56 NATO: What You Don't Know Could Hurt You - Bearing Arms - Remington

TIA
Mike

S&W M&P 15s are chambered for 5.56mm NATO cartridges. You can fire either 5.56 or .223 safely.

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CSM, U S Army(Ret) 1963-1990

.226 Wylde and shoot both

Mine seems to shoot .223 more accurately (I should say I shoot it more accurately) plus it seems to be cleaner. That, in addition to it being a little cheaper, makes me prefer it to .556. I shoot plenty of both tho....

As mentioned, you can shoot both. I prefer 5.56 to .223 because I have a ton of 5.56 and both of my ARs are zeroed for the 62 gr.