I recall reading that some of the early big game hunters prefer to shoot solid slugs out of a smooth bore gun usually in the larger calibers . Could this be something similar . but for smaller game plus the advantage of shotgun use > Makes me wonder about a 410 I have in the shop at present that also has extremely heavy barrels . Your gun was almost certainly made for colonial use ,Africa or India so may have been envisaged as a general purpose shotgun /rifle .