The Makers still exsist and so does their records: http://www.wrichardsguns.co.uk/index.shtml They may be able to help with your query. If it is the original case then that will show the correct barrel length. Whilst 26" 20 bore hammer guns are not common they do exsist. I have a Jackson of Nottingham with damascus barrels proofed for black powder that has original choked 26 inch tubes. 30 is by far the commonest length. Lagopus.....