Chamber length Win. 21

[Cold Spring]

I'd say it depends on Winchester's chamber specifications when your 21 was made. Earlier on, makers were using their own 2-3/4" specs. For example some Foxes were made with tight chambers though on size as regards length. There's no date on that print you posted but you said it's current SAAMI for 12-gauge 2-3/4" and presumably you are or will be shooting modern shells. Hence a .798" diameter pin gauge should let you verify your gun(s) chamber length relative to that print.

Yes this has gotten too involved. My last reply here.

[Bill Murphy]

The beginning of a forcing cone is a visual sighting at the end of a machinist's scale. An alternative is the movement of a dial on the end of a choke gauge. The actual dimensions are rather irrelevant as long as your loads are safe.