King carp wrote on Apr 4^th, 2020 at 3:57am:

. . . does the monofilament reduce case head expansion? I had never heard of doing this.

The monofilament serves as a spacer between rim and breech face, holding the case head tight against the bolt face. This can greatly reduce the axial stretch that occurs when the case body adheres to the chamber during firing but the non-expanding solid head moves back, taking up any slack due to clearance between case and bolt. (Animation shows stretch/separation on firing with excess clearance and no spacer.)

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Because the spacer is located at the solid portion of the case head, it does nothing to prevent radial expansion of the case body forward of the web. Normally this radial expansion is a problem only with grossly-oversize chambers (or badly undersize cases) and then only at high pressure (see picture below).

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