I see Butch has spilled this over to AR:
http://forums.accuratereloading.com/eve/forums/a/tpc/f/9411043/m/6531089131/p/1
There are no right answers.
We come away with what we want to come away with.
What I got:
The C ring takes the force of the breech as a cantilever. It is compliant.
The large ring takes the should one end in compression.
Hit either one, and the the barrel can be turned further, but with the large ring, there is no measurable axial movement, just rotational as thread crush compliance allows the last turning.
Why does it matter if the C ring is compliant?
It affects a few things:
1) If the C ring is compliant and one wants both the large ring and the C ring to make contact, then the C ring makes contact first.
2) If the C ring is compliant, it can act like a lock washer.
3) If the C ring is compliant, and one wants the soldered on open sights to clock straight up [interchangeable barrels] then margin for allowable error in clocking is over the range of not only thread crush, but has increased to thread crush PLUS C ring cantilevered bending.