I have never been able to figure out why everyone calls Unique "dirty". I've used a lot of different powders and have never noted Unique being any "dirtier" than any of the others. I think one of the main problems is not the powder but those who use it... A lot of shooters lightly crimp their cases so as to increase case life...this reduces bullet pull which isn't conducive to good burning... I "crimp the crap" out of all loads and have no soot, unburned powder or anything else to speak of regardless of the powder used.
The powder I've had the most problem with leaving unburned powder behind is everyone's darling...2400. But then it may have been because the loads were not heavy enough...
Bob
I never considered Unique "dirty" either. With jacketed bullets it seems no worse than any other powder as far as I am concerned, and certainly not troublesome.
Cast bullets, especially with soft Alox lube, are very dirty to shoot and cake the revolver up with all sorts of grey crud. Unique is very commonly used in cast bullet loads, so I think the powder gets blamed for the sins of the bullets.
