4ga = 1.052"
5ga = .976"
6ga = .919"
Unlike most gauges chamber dimensions for the 4 gauge were set using thin brass shells, not sure but probably as a rifle. For paper case 4ga shotshells, chamber remained same size but bore size was reduced to about .935". Since both nominal chamber chamber size & in-between bore sizing (6/1 etc) were introduced in 1887, any 4 ga shotgun for paper cases proofed in England prior to 1887 would simply have a "6" stamped under the bbls. After 1887 a 6/1 bore size would have been .938" so many would still be proofed as a 6ga, but would have the 4/C in diamond chamber mark. One wonders if someone did not confuse a pre 1887 gun proofed as a 6ga but having 4ga chambers as a "6 Ga Shotgun".
