Hunter Arms Co. lengthened 20 gauge chambers from 2 1/2" to 2 3/4" in 1936. L.C. Smith and Fulton 16 gauge guns manufactured as late as 1939 are still found to have 2 9/16" chambers.
Unless otherwise marked, 12g chambers from the time of L.C. Smith Maker, Syracuse hammer (1884) and hammerless (1886) were 2 3/4".