If it's a headspace gauge it's likely 7.62x39, but without any markings it's hard to say if you have a go or no go or field. The 54R is just a little rim.
But maybe it's some other kind of gauge like case internal length? Photo would help.
Sorry it's not in focus. It has 7.62 Russian printed inside the band. I measured it to be 28.8 mm long. The inside measurement is tapered from 12.3 mm to 11.7 mm on the small end.
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