Bolt to tenon clearances serve some less than obvious purposes. Metering of expelled gas and debris as well as extractor containment, in the event of case rupture are a couple. Also related to this is the ability of the bolt nose to expand to fit the counterbore in extreme overpressure instances. The depth is also critical for these reasons as well as to prevent too much unsupported case.
I read years ago that the Remington "three rings of steel" was designed with .003"-.005" clearance on these surfaces as being optimal. I agree with Mr. Stiller's drawing.
We could debate reliability issues with tighter tolerances, but they don't really matter if your face and eyes are full of brass.