Hubert, if your double was made in Europe, it was very possibly a "becassier," or woodcock shooting gun. French and Belgian makers made these for hunting woodcock, which are typically shot at about 0 feet from the shooter, and require very small shot and a very large pattern if you are to both hit them and also not blow them to bits. Woodcock ("becasse") are a prized bird made into special dishes--sort of the truffle of upland game-- and hunters didn't want to waste any, hence the specialized guns. The straight "rifling" helps disperse the shot into a wider close range pattern.
The other barrel is used for longer shots and for other game, and the "rifled" barrel was also used for close shots at other game like rabbits. Often a "becassier" had short barrels, around 25" but a metric measurement.
If your double is American made, then I'm stumped!
I should of kept it I guess but I was poor and young at the time. I tripled my money for what I paid for it..
