One piece gage that will check overall length to indicate the need for trimming,
datum to head length to assure correct headspace and to prevent over sizing.
A must to have. I purchased from Wilson Combat a bag of once fired brass and used one to gauge how much material to remove. I set up my Hornady case trimer to stop at the correct length, then put a drill on in place of the handle. I was putting out perfect cases as fast as I could pull the trigger on the drill. You have to own this gauge if you want to make perfect cases.