Headspace...

[Son]

There are a few makers of headspace gauges that are making them to saami specs (the spec they made up) but not to .303Brit specs.

The GO gauge is .064", I believe, for all makers I have seen or heard of.
Forster gauges are .064 GO. .067 NO GO and .070 Field.
Milspec is .064 GO, .068 NO GO and .074 Field.

The middle gauge only exists in the factory. It is the spec for setting the bolt head in new rifles after proof firing. Every rifle that has been issued is tested by the .074" Field gauge and passed for service until it fails the .074" gauge.

Check the brand and size of the gauges you are using. Regardless of what they are, passing the field gauge means your rifle likely has many thousands of rounds left in it as is. Even more if they are the false Forster gauges.