First off, I don't load 40 S&W and I have never used either Blue Dot or Longshot.
Both are slower than the beloved Unique and Longshot is slower than Blue Dot. So, for those who really like slow powders, these ought to be standout powders.
The thing I like about the idea of Longshot is the wide variation in charge weights and the range of velocities. The Hodgdon site has the loading data for Longshot and the 155 gr XTP bullet.
I would start with the Longshot (I have heard it is usually called Loudshot) and make up a few at various charge weights. I don't think I would go all the way to the high end; perhaps I would stay 0.2 or 0.3 gr away. There is so much ground to cover, my increment might be 0.2 gr.
I wonder how much flame is going to be following the bullet. If possible, I would try to shoot some at night.
But if I already had the powder, I would certainly like to give it a try. Even the lightest published load with Longshot makes 'major' - pretty close to 175,000 power factor.
I have been considering loading for 40 S&W and I have been looking at the Longshot data for a while now. After you try it, please post your results.
"No matter how cynical you become, it's never enough to keep up." - Lily Tomlin