Re: headspacing an AR
The difference between the two chamberings is the throat, which has no bearing on a headspace gauge. That said, headspace gauges from different manufacturers can vary a couple of ten-thousandths and there is also a couple of ten-thousandths difference between military headspace gauges and civilian headspace gauges, both marked 5.56x45.
It's all hooey. Temperature buildup as you repeatedly fire the rifle can make that much difference in headspace.
If the barrel extension was installed by the barrel manufacturer and the locator pin installed the headspace is fixed. We periodically check headspace on our Standard Series builds (where we purchase barrels with extensions already installed) but have never had an issue. Never.
80 lbs of torque is the recommended setting for GI armorers installing GI-style barrel nuts. If you are installing a free-float rail system, you will be using between 35 and 50 pounds of torque. Use plenty of anti-seize and tighten/loosen/tighten until you get the gas tube clearance in the desired torque range.
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