20 gauge chamber dimensions

I am examining a 1923 British 20 gauge shotgun and am asking for help re; maximum chamber diameter, maximum bore diameter and minimum wall thickness. TY ahead of time for your consideration. krkdds

welcome krkdds
we are mostly interested in ammunition & not so much the firearms here, so perhaps another more firearm related forum might be more productive?

I’ve looked in my 1925 copy of RULES OF PROOF, Birmingham house and the only information relating somewhat to your ?'s is; 20 Ga.= “.615” diameter of the bore".

The Birmingham Proof House has a web site, perhaps they can help?

because it is from before 1925 here are the dimensions


“Thank you Sir to have spent time and energy to give me the info I needed, despite the fact you don’t know me”

The politeness is a value which gets lost.

F. Y.