Randall - On the 12" barrel, the gas port is .006" bigger than the 10.5" and the round protrusion is .003" less than the 10.5". I will get pics tonight as there is no way I can explain what I did.
Printable View
The only reliable way to measure is against the barrel's torque shoulder or by comparing against a known standard like a headspace gauge.
http://ar15barrels.com/tech/ar15bolt-extension.gif
Measuring from the rear face if the extension is not reliable as the length of barrel extensions can vary.
If you had a surface plate and a tall enough height gauge, you could work out the messurements from the front face of the extension.
This would be accurate enough to compare headspace dimensions as long as your bolts are roughly the same.
I have developed my own simple gauging tools that I use during chambering.
http://ar15barrels.com/gfx/gauging.jpg
Thanks Randall for the always cool pics.
I broke away from work today at lunch to go shooting, but this time I took both Silver Bear and some old lacquer Wolf ammo. I also removed the extra power extractor spring, black bumper and o-ring from the bolt and replaced it with the original extractor spring and blue bumper. Well...I had success for awhile.
I ran through about 100 rounds of Silver Bear before the empty got stuck in the chamber. I tapped it out then ran close to 500 rounds of the old lacquer Wolf without a single problem! I wonder if the spring, bumper and o-ring had anything to do with the problem I was having. Granted, an empty still got stuck, but it did it after 100 rounds or so. Before, it was happening every 2nd or 3rd round. So maybe a combination of ammo and extra power extraction? I don't know what to think now.