As for headspacing:
Just put the barrel (with barrel extension on it) into a vise (with padded jaws) and then get your bolt and carrier group. CLEAN THE CHAMBER. Make sure there's no dirt or crap in the chamber - check it with a flashlight.
Put in the go gage. Put the bolt (extended out from the carrier) into the extension. Push forward.
The bolt should rotate closed completely.
Pull the bolt backwards. Pull out of the extension. Pull the go gage out of the chamber, wipe it down and put it back into the container in which it came.
CHECK THE CHAMBER again for dirt, dust, etc. Put in the no-go gage. Extend the bolt from the carrier and push the bolt into the extension again. Now the bolt should NOT rotate to full closure. You can tell this because the carrier will not be all the way up to the back of the extension - it should be sticking off the extension by 1/16th to 1/8th of an inch.
Pull out the bolt, pull out the no-go gage, wipe down and return to container.
The only time I've seen this fail is when someone put on their own barrel extension and they did not torque the barrel extension down onto the barrel tenon with a torque wrench or the proper tools - which then allows a bolt to close on a no-go gage.
If you're putting on your own barrel extension, then get a proper extension driver to put into the flutes of the extension, and get a 1/2" drive torque wrench. The spec torque is 150 ft-lbs. Do not skip on this torque. Get your barrel into a proper vise with proper lead soft jaws, or you need to get some aluminum or plastic blocks around your barrel into a vise without soft jaws. If you're using plastic or aluminum blocks to hold your barrel in a vise, you might find that you need some powdered rosin to keep your barrel from slipping as you torque on the extension.