Headspace gauges for 30-06 are totally interchangeable with 35 Whelen.
There has never been any confusion on this subject until the Whelen became a SAAMI registered cartridge. Why does that cause confusion? Because people fail to notice important details.
Above is the 30-06 Springfield SAAMI chamber print. Note that the Datum point for the 30-06 is .375″ in diameter. This is the defined point along the shoulder where headspace is established. We do this because the shoulder is at an angle, so to establish headspace you need to be able to set the length of the chamber from the bolt face to the datum point (this is precisely what headspace gauges do).
Above here, is the 35 Whelen chamber print from SAAMI. Note that the Datum point for the 35 Whelen is .415″. This is necessary because the .375″ diameter does not exist on the 35 Whelen shoulder. Shoulder angles are identical, but we are measuring at a different point along that angle. Another way to say it, the angled shoulder on the gauge is in exactly the same location on the 30-06 and the 35 Whelen. Details matter.
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