This video is problematic shouldn't be held out as an example of how to properly verify headspace. It flat misses the mark. Just like a micrometer or electronic calipers, there's an art to taking a measurement with a headspace gauge. To specifically address the problems in the video, there are two issues that stand out. To properly and accurately verify headspace:
1. The barrel should always be removed from the upper receiver.
2. In conjunction with the headspace gauge, a master bolt should be used to verify headspace.
There are three parts that form the chamber on an AR-15 pattern rifle. The barrel, barrel extension and bolt. Using a used bolt to verify headspace, as is done in the video, doesn't eliminate the bolt as a source of an out of spec reading. Using a max reject gauge to verify service life of a barrel is the only gauging I subscribe to. The go / no-go gauging is realistically a manufacturing quality assurance process and really has to be done before the gas port is drilled. It's gonna be expensive to try and repair a new barrel that actually fails gauging.
