Yes. If he has a mandrel die body or some other means of expanding the brass before he runs it into the die, that will quickly reveal the neck's diameter. I use the half ball split expanding type small hole gages for this kind of thing. But as with any tool, it only helps if you have it on hand. It occurred to me belatedly that he could have slugged the neck of the die, too. But I hope he'll tell us what CH4D says?