So my cool-as-grits S&W M&P Sport II is of course chambered 5.56.
I've been shooting .223 only out of it ( 700 rounds or so ) cuz the ammo is available blah.

So is it cool to just shoot .223 out of it or should I mix up some 5.56 rounds now and again or only shoot 5.56 because that's what it is chambered for.
What say.

I love shooting this rifle it behaves like I want it to, it produces nice groups, it hasn't failed me once.

It will shoot either .223 or 5.56 without issues. The main difference between the two is that 5.56 has higher pressure.

5.56 NATO and 223 wylde chambers can shoot 5.56 NATO and 223 rem.  223 Rem chamber should only shoot 223 ammo.

you're fine.

Test zero on different loads.  Don't expect 5.56 and .223 to have the exact same POI