Somewhere I read an article on this, but off hand don't recall where, I think perhaps in a shooting sportsman mag from several years ago. Technically a 2 ga is 1.325" dia & a 4 ga 1.052" dia. In actual practise most 4ga guns are built for a paper case with actual bore dia of about .935" & proofed as a 6ga (.919" Dia). For many years, the author stated, 2 ga guns had been built with actual 4ga bore dias of 1.052 & Proofed as such but chambered for a paper case of larger dia the 4 ga chamber dimensions & listed as a 2ga gun (the 4ga chamber size was established using thin wall all brass cases). You should be able to find a machinist or gunsmith around who could measure both your bore & chamber to know for certain what you have. This would be most highly recommended as a first step with anything of an unusual nature.
