In selecting a "clean" powder, one thing that you should keep in mind besides operating pressure is load density, particularly with autoloading pistol cartridges. Choosing a powder that fills the case to the base of the bullet is desireable for several reasons, not the least of which is that it prevents overcharges but more importantly helps check bullet setback in case of a poor crimp.
I first started considering this when loading for 357 Sig, a somewhat more difficult auto pistol cartridge to load for. It has a short neck, the bullets often are just beginning their ogive curve at the point where crimping for max OAL is reached, and the bottleneck itself makes proper crimp setting difficult unless done in a separate step. Having a case nearly full of powder helps to prevent setback during crimping, and may aid in lessening it as the bullet is driven into the feed ramp.
I think most of the problems I have seen with 45ACP, for example, came with the use of low load density powders coupled with a poor crimp. If setback occurs, you could go high order. In the 357 Sig, it can become a significant issue quickly. So in trying to get the "cleanest" burn, be cautious against selecting a low load density fast powder that leaves a lot of air in the case. It is better to pick a powder that burns better at its preferred or max operating pressure but provides a full case than one which is low loading density, imho.
Of course, with cast bullets, some powders burn "hotter" or have progressive burning characteristics that are slower to reach max pressures to give good obturation and seal, thus cause volatilization of the bullet base and lead redeposition in the throat or first part of the bore. Thus, bad leading. That is one of the reasons you will see that I rarely use or advocate Unique any longer. I had far more problems getting it to do what I wanted without a lot of leading, especially with variable hardness commercial cast bullets, than other powders that burned "cleaner".
