Binko,
English guns proof tested and stamped 11,11/1,10,10/1,10/2. were all chambered as 10G; no size is given in the rules of proof for 11G chambers.
The same Logic applies in other gauges. We see many vintage 12G English guns that were proof tested and stamped 13G but in every instance the guns are chambered as 12G. there being no 13G chamber sizes in the rules of proof.
I have seen 12G guns with bore diameters up to.750 which is the maximum diameter for a 12G proof tested and marked 12/1
It might well have been legal to proof test a 12 G gun bored .729 as an 11g provided that the gun with stood the 11G proof test. Suggest that you contact the Birmingham Proof House for advice.
