If you need low-pressure shells for something, it might be best to get into reloading for it, and it might be best if it is a 12-gauge. Damascus guns if safe, would remain safe with black powder loads, fluid steel 12 gauge can be loaded low pressure with modern components. I have found that trying to load low-pressure for 20 and 16-gauge cartridges to be frustrating. 12-gauge brass shells are available for purchase and hand-loaded without machinery. I haven't done it, but there are articles online about it.
Have you looked at Hodgdon's Long Shot powder? I believe I used it for some low pressure loaxds in a sixteen gauge damascus Evans hammer gun. It is readily available.
