Miller, all I was saying was that if the 2400 didn't work up to the pressure, sound, and performance level with a reduced shot load, your recommendation of Unique is not a proper recommendation because there are at least two Hercules powders between 2400 and Unique that would be better choices because they are progrssively faster burn rates than 2400, but not as fast as Unique. Unique is at least three steps up in burn rate and potential pressure level from 2400 and shouldn't be the first choice to replace 2400 when used in a black powder era .410 shotgun. Your anecdotes about early day usage of these powders and various uses in loads unrelated to the light .410 are very entertaining, but not too pertinent to the situation at hand. Your last sentence makes me wonder how you know that a 3/8 ounce .410 load propelled with a slightly reduced (under standard 1/2 ounce target load) of 2400 would fall below 9000 psi? In my opinion, such a load would probably be just fine and would have a pressure of maybe 9500 to 10000 psi. I, personally, would try it before switching to a powder three levels faster, especially in an older black powder era gun.
