One of the steps in accurizing an AR-15 is to reduce the tolerances between the barrel extension and the upper receiver. This is most commonly done by wrapping the barrel extension in .001" shim stock and then reassembling the rifle, or by installing a larger diameter barrel extension sized for a press fit in the upper receiver. Since SIG controls all production tolerances on the guns they build, how does SIG address this issue? Is there an effort to make the fit between the the barrel extension and the upper receiver as close as possible? If so, how close are these tolerances?

I've been here about as long as you have. I have never seen a person qualified to speak for SIG about a production question answer a question. OK that said. I own a bunch of 716's, so I can probably assure you they don't do squat about tuning anything.