Chambers were normally the same for both. Some early guns made especially for brass shells had oversize bores. I have heard of, but never encountered, a few very early guns made with under size chambers for a special brass shell which would use the ordinary 12 gauge wads rather than oversize wads. As the ordinary brass shells with their over size wads could be readily used in guns having standard bores, this alternative didn't last long.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra