the 37 rounds are not DD until they are over the allowed pwder charge or loaded with lethal or non lethal projectiles . You can register the launcher as a DD but I think you would be better off registering the 12ga adapter .
I have loaded some 26.5 shells with some pyro but I am too scared to fire them off or at least till I get over open water , they are ..... experimental rounds and I have no clue if they will go pop or boom or kaboom , my luck they would make it out of the tube and fall on the ground then ignite .
When I get some time I am going to get back into it and learn some proper load data and get a handfull of reloadable 37 cases .
I can buy 26.5 orange russian surplus smokers but they are boring , huge fire hazard and over 5.00 a shot .
They are not designed for high pressure rounds but mine is a B&D and is very stout , I would keep the 37's low but once I get the adapters in I would have no problem bumping up the pressures , I know the breech will hold just fine and the extra 1/2 in thick aluminum adapter ontop of the already stout barrel will hold up nice for what I would want to do with it