we need basic informations: gauge, left hand twist ( french military rifles have this) or right hand twist or straight rifling, chamber lenght, proof marks, one or two rifled barrels

best you made pics of the whole gun with any detail that could help. could you measure the land and field diameter of the rifling? if being a normal gun with a rifled barrel for optimising the use of slugs you could probably take any slug of this gauge but ... most slugs have slant ribs around the body and if this is in the direction of the twist it dont work very well

you see here a common Brennecke shot gun slug, the rips turning into the right direction like most rifling does

here is a special Brennecke slug for rifled barrels where the rips turning into the left direction and the rifling get them