It sounds like you have a headspace gauge. A throat gauge looks like this:

http://www.fulton-armory.com/%5Cfaqs...%5CTEGauge.htm

It does not need the bolt to operate. Headspace is from a datum line where the shoulder is 0.375" to the bolt face. Throat erosion is ahead of the case neck and is the diameter of the throat or leade, wear from firing erodes it. M1icon Garands have a longish throat compared to the M1917 or M1903/a3 rifles.

Dave