There was a magazine article on the 223 45gr WWB ammo a while back, I think a few years ago. I've seen it posted on a shooting forum somewhere. I saved it, but then lost the link when my computer crashed.
In summary, it said that the bullet used is Winchester's own creation but it is not available except in the WWB pack. There was supposedly a lot of research put into the design, which is why it is so accurate. The bullet is constructed with a large cavity inside which helps it blow up. It's typically a great bang for your buck, of course not all barrels like it. Mine doesn't. But lots of folks have good luck with it.