J, your example of chamberless fowling gun is not of much use for the question asked. Chamber-less gun are relatively rare and not a standard chamber per the question asked.

2 piper hit it on the head. The relative chamber sizes were not different for early Breech loaders whether they fired all brass and paper and brass hulls. While some chambers are narrower than others it is not because they are meant specifically for brass hull..

Michael Dittamo
Topeka, KS