Now what I don't understand is why it would not work?
Because explosions (and ballistics) are often exponential. I reloaded for years, pushed the envelope significantly..I used to take my primary 44, a Redhawk, to the smith every couple hundred rounds to be examined. I've pushed a 300 grain 44mag to around 1300fps. Yes, it was a handful, but I put down a doe at app. 174 paces with one shot. I shot some pretty hot ammo fairly often, yet I would advise against shooting the 38 rounds with the magnum powder charge. Just my $.02 worth...............
And yes, Unique is a fast burner...and was always one of my favorites. Very versatile powder. Made a great shotgun load too.