A .223 Wylde chamber is basically a hybrid design between 5.56 NATO and .223 Remington specs. It has tighter tolerances than 5.56, but larger than .223, so it’s basically the sweet spot between the two. You can safely fire 5.56 NATO and .223 Remington ammo through a .223 Wylde rifle. Here’s a helpful video from Brownells explaining this in more detail.