Not everyone is convinced that bore size or forcing cone length matters; including Neil Winston
https://www.trapshooters.com/threads....365553/page-4
Neil Winston “Have Today's Increased Bore Sizes Improved Trap Gun Performance?” 2016
https://web.archive.org/web/20180829...e_Diameter.pdf
And for a historical perspective, the chamberless Tolley "Altro" was introduced in the mid-1890s
https://books.google.com/books?id=BF...AJ&pg=PA31&lpg
https://books.google.com/books?id=Bz...AJ&pg=PT92&lpg
Dr. Charles J. Heath chamberless patent
Forest & Stream July 1921
https://books.google.com/books?id=x1...J&pg=PA308&lpg
1922
Sporting Firearms of Today in Use
https://books.google.com/books?id=9X...AJ&pg=PA95&lpg
Field & Stream, August 1922
https://books.google.com/books?id=XQ...J&pg=PA470&lpg
A few Lefever have been documented with "Taper System of Boring"; gradual constriction from end of chamber to the muzzle.
Forest & Stream 1909 "First to adopt the taper choke system."
https://books.google.com/books?id=l0...J&pg=PA395&lpg
Charles Askins on "taper choke" in October 1917 "Recreation"
https://books.google.com/books?id=gX...=RA1-PA210&lpg
Daryl Hallquist has confirmed that an occasional Lefever 12 gauge has bores tapering from the breech to the muzzle (.756 to .708 left and .715 right on one gun). Most have a standard U.S. .729 - .732" parallel bore.