You do not measure cases to determine Head space , you use these
https://www.brownells.com/gunsmith-t...uges/index.htm .
Military chambered rifles are generally use bigger dimensions than civilian rifles.
Milsurp cases are generally thicker .So you tend to use lower powder weights.
Here on pg42 you will find 6.5x55 case dimensions
https://saami.wpengine.com/wp-conten...sting-Copy.pdf
First resized case is on the small size .
Basically your cases are of unknown age / stored for a long time in unknown conditions / Used in a Milsurp chamber / Used how many times ??. Candidates for the bin , they are worn out.