Ok I think I understand, I thought that slipping a .366 bullet into the case neck was to confirm brass OAL.

If I understand you correctly the method to this is, since the .366 bullet slips into a fired case means the the beginning of the rifling is wide enough to accept a bullet of .366 diameter. If it was too narrow and thus a different caliber/groove diameter, the case mouth would not be unable to expand all the way as there is not enough space in the chamber for it to expand and thus the case mouth would remain narrower than the bullet diameter after firing.