Your rifles bolt SHOULD NOT CLOSE on a field gauge at any time!!!
The true name of the field gauge is FIELD REJECT GAUGE, at no time should you fire a rifle that will chamber a field gauge. Because your rifles bolt will not close on a field gauge DOES NOT mean that it has short headspace.
When your bolt fails to close on the GO gauge, then you have a short headspace condition.
The above statement that a case seperation is indicative of long headspace is true, but also be advised that if the bolt does not close completely due to short headspace, the weapon is still capable of firing. This is more or less an out of battery discharge and can cause a case seperation also.
Headspace is normally set between GO and NO GO, the FIELD REJECT GAUGE was implemented to give depot level maintenance personnel an easy way to determine the servicability of a barrel.
In an issue weapon, the headspace is set at the manufacturer, once it goes in for maintenance the field gauge gives them a quick idea of the weapons suitability for reissue. If it takes the field gauge, it is done, finished, over. Unless of course it is a time of war and weapons are in short supply and the commies are in the wire and every weapon is needed like right now!
Chuck