eBay is your friend. Pick up a name brand all-steel digital caliper. Ensure that it goes to zero each time, from 106, and tell them so. Your written words should be something to this effect:
I have been trained in the proper operation of precision measuring tools
The digital caliper used in these pictures is a (make, model), serial number: (serial), which is a high quality measuring tool suitable for the presentation of evidence in court
This digital caliper provides consistent measurements and returns to zero each time (photo)
Add whatever court experience or years of firearms experience you have. Any relevant courses, eg. Photography, machining, commercial law in high school. Dig deep.
Get yourself 10 minutes worth of training from a machinist on how to make precise measurements, so that you ARE trained. And don't forget to end your very formal and carefully written email with the words indicating that you are prepared to provide this as sworn evidence in a court proceeding. My favorite conclusion is "respectfully submitted", provide your full name again and contact information.
You can probably borrow a quality caliper somewhere, but it's a bit harder to dismiss a man who shows up with his own professional tools.
Send this, CC the people involved including your verifier, and do this with the expectation that the right thing will happen. Persist, because you are right.
And don't forget to tell us your good news when you finally succeed!