Just got clymer .308 gauges. It's the first time I've bought or used gauges. I was surprised to see that one side of the base was cut at a 45° angle. What's that for? From what I read I'm supposed to put the rim under the extractor and try to chamber it. I'm planning on cutting a relief for the ejector instead of removing it. The angle they cut doesn't seem to accomplish anything. What's it for?

Since your not supposed to full speed bolt release the gauge into the chamber...

That cut-out is there for your extractor to pass the rim easily when you gently push the bcg into battery.
Also i found out recently that no ejector cut out or removal is necessary to get an accurate reading

Everywhere I read said to put the rim under the extractor and leverage it over to get it in the chamber, I've never seen this cutout mentioned.

Just explaining it as it worked and how i saw it for my 556ers and clymer gauges. When bcg either fully engaged, or did not on a no-go... the gauge gets pulled out of the chamber via extractor. So to me it is only a matter of having the cut out lined up, and that is all it has ever been checking 3 chambers with 2 gauges