This is all I have, and things may certainly have changed between 1914 and the publication of Jack O'Connor's The Shotgun Book in 1965
16 gauge bore and choke
……..…..……Browning……….Remington…….Winchester
Bore………...…0.665”…………..0.673”…………..0.664”
Full Choke…..0.037”…………..0.033”…………..0.031”
Modified….….0.025”…………..0.017”…………..0.016”
Imp. Cyl..…..0.016”…………..0.008”……..……0.007”

Certainly the British standard for "Half-choke" would be 1/2 of the Full constriction