HD_Ride
I own Mosins and Enfields and both are rimmed cartridges and I also have headspace gauges for both but I'm going to show you how to check headspace without gauges and check it accurately.
With a set of vernier calipers measure the overall length of a new case or a full length resized case and write it down.
Now take a spent fired primer and start the primer into the primer pocket just using your fingers.
Now chamber this test round and slowly close the bolt fully and "seat" the primer you put in with with just your fingers.
Now eject the test case and remeasure the overall length again and write it down.
Now subtract your first case measurement from the second case measurement and this will give you head clearance (head gap clearance) or the "air space" between the bolt face and the rear of the case.
Now measure your rim thickness and add your head clearance measurement to your rim thickness and this will give you your exact headspace measurement.
Example: Rim thickness .058 + head clearance .011 = .069 actual headspace measurement.
What you are actually concerned about is head clearance or the "airspace" between the rear of the case and the bolt face. this "airspace" is what will cause case head seperations if the "airspace" is too long. (excess headspace)
7.62x54R American SAAMI headspace settings
.064 GO
.067 NO-GO
.072 FIELD
Russian Go = 1.60mm = 0.063"(0.06304")
Russian No-Go = 1.70mm = 0.067"(0.06698")
Finnish Go = 1.63mm = 0.064"(0.06422")
Finnish No-Go = 1.73mm = 0.068"(0.06816")