Longer throat,with slightly different angle into the rifling/bore. Look up the 223Wylde chamber drawing and compare to these two.
I have shot thousands of 5.56X45NATO thru 223Rem chambers . . . HOWEVER . . . nobody else should.
I’ve got one , a Wilson 18 inch . It’s pretty accurate. But it’s a premium barrel and I believe it would be accurate even if it were chambered in 5.56.
The wylde was designed to handle the pressures of the 556. The idea is that you can accurately shoot 556 and 223 out of the same barrel. Whereas with a 556 NATO barrel would be less accurate when shot with 223