"Paper case " 4 bores are only nominally a 4 gauge.These are generally stamped as a 6 bore (0.919"), although strictly speaking the" standard bore" diameter of 0.935" is closer to a 6/1. The diameter at the lower end of chamber is 1.035" - ie there is 0.100" constriction leading from chamber to bore. The wall thickness for a paper case, Eley Kynoch, is 0.045 "(... compared to 0.027" +/- for a 12 bore paper case.)
Brass case 4 bores are true 4 gauge in the barrel - 1.052" - which according to Greener will carry an equivalent load to the paper case 2 bore. Accordingly these guns will not carry the same amount of constriction from chamber to bore if they are designed for thin brass cases. I think that the smooth bore rifles used thicker walled brass cases and were likely in the 6 gauge range of dimensions.
Obviously there was much variation of bore dimensions in the four , eight and ten gauge categories -apart from chamber lengths !!
Additionally, many 8 bores and not a few 10 bores have undergone chamber "adjustments" - carried out by the trade to the request of later owners, an often ill-advised procedure.
