...Here is the extension diameter measurements and the a measurement from the Barrel base inside the extension to the front of the extension bolt lugs. If the measurements in the photographs don't look like they are exact it's because I am somewhat handicapped having only two hands. It's hard for me to hold a ruler, flashlight, camera and barrel all at the same time. I measured using a strong magnifying lupe and they are with in a couple of thousandths, if not exact, I would say. I going to assemble in a few days and test fire.
Anyone know of a source for AR10B upper receivers by the way?