The fact that it's incompletely burned powder is no question.  In fact, when shooting over concrete (as we do at Kalamazoo), you can see the powder spray over the concrete when shooting prone.  When I refer to "Dirty", I'm really referring to the garbage left in the barrel (black sooty gunk) which takes about twice as much cleaning as my 1903 takes with the H4895 loads I shoot in her!  The unburned power is the real reason I'm planning on switching.  I feel that a 100% burn can only contribute to accuracy... and I really want to improve on that for next years matches.

By the way, thanks for posting the data, which helps to illustrate to everyone the advantages or disadvantages of one powder over another!