Gauges are available.
http://theguns.esportssale.com/+7+62+X+54R+headspace7.62x54R = Rim: = .064" - Headspace = .064" - .068" (Hatcher, p. 436)
Other sources cite the rim thickness as 0.063". The difference may be due to conversion from metric to English dimensions.
Hatcher also notes that in
rimless cartridges, excessive mechanical headspace is not all that critical in terms of safety unless there are other things wrong such as a soft case head. He also notes that measuring headspace involves removing everything from the bolt head that might interfere with the measurements. He notes that in many cases
rimless cartridges may be driven into the chamber, even manually, with enough force to create "excessive headspace" by resizing the case in the chamber.
If it were me, but just for me, I wouldn't worry about it too much, and I would just go ahead and fire it with the standard precautions about eye and hearing protectiom and checking the firing pin protrusion with that funny gauge that came with the rifle. But that's me.
I seem to recall from the last time I took my bolt apart, that it is tricky to re-assemble it correctly, and I vaguely remember that it was possible to assemble it in a dangerous way. But it's been a long time since I took it apart.
You could also (with the range officer's permission, of course) mount it in an old tire and fire it a couple of times with a lanyard to check it out, and exmine the cases for stretch rings near the web of the case.
More to bump the thread than anything else.
Terry, 230RN