tinker

with my limited knowledge about this topic I would say it was made for a paper case with a heeled bullet. believe that purdey had have a very small case when they make a chamber reamer. nothing was standardize in this days and we will find also today that shotgun cartridges have a lot of different measurment. what was in 1862 with cases from france, belgium, germany or brittain.
the chamber is to small? not unusual for an early gun. I haver a simple 24 ga Colette/Liege hammer gun with very tight chambers, beliefe it was make before 1880. the gun makes absolut no problems with old paper rounds from Eley, RWS etc. also the new Fiocchi plastic will fit but Magtech 24 ga brass want.
I was thinking a second about a new chamber reamer and this will be history but decide to rework the brass.

look at the pic, I turn the brass over the rim ( believe 0,3mm) and size with a .577 snider die. so it is possible for the gun to take the brass.