Hello Fastbolt:
After a Google search led me to this thread (as I was curious if an internal extractor tension was checked different than external one) I put this gizmo together using 1/16 X 3/4 inch brass strip:
The piece against the breechface and under the extractor was made the same width as a mid-spec SAAMI 45 Auto case rim, or 0.475". I measure tension at the instant the edge of the strip opposite the extractor begins to move away from the breechface guide block - detected with the help of a magnifier.
Is this about how S&W does it?
Regards