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This is an excellent company to deal with. I have their 303 gauges and they are a very good product.
For the purposes of a Mosin, I would just get the field gauge.
http://www.okiegauges.com/
Headspace gauge for a mosin nagant, tie rifle to bench or tree load round, tie long rope to trigger step back and fire.I do this with any old rifle I buy, even if the headspace was checked.
Brought both mosins out today. Both fired, no broken peices and im still in one piece. Only issue i had was with the 91/30 loaded 5 rounds and the first round didnt want to chamber. Solved this issue by pointing the barrel down and seemed to help.
I have a set but I am not near Essex county. Just remember if you do get a set you have to remove the extractor to have it work correctly. If you don't to take the chance of not getting a proper reading.