.223 vs 5.56 for the millionth time? (Well, sort of.)?

My question is specifically geared toward Bushmaster AR's because I've never owned any other kind. It is marked ".223-5.56mm." No problems so far. But from the reading I've done (could be wrong) .223 is less accurate when fired in a rifle marked 5.56mm. So, does anyone know if Bushmaster just uses 5.56mm chambers? Or do they have some kind of proprietary chamber that allows for .223 to be fired just as accurately as 5.56mm?