I think that what seemed so dirty to me was the residue left from the lube on the bullets themselves and being burnt by the next round that came through.
If that's what it is, it means the lube is working well. A lubed bullet will leave a lot of its lube in the barrel to lube the barrel for the next shot, the lube keeps the lead from the bullet from soldering itself to the bore. Cleaning lube out is easy and beats the heck out of trying to get lead deposits out.
BTW, if you get any leading- it looks like grayish streaks in the grooves, get a 'lead removal' cloth, cut patches out of it and use them to clean the lead out of the barrel. To prevent this from happening in the future, try dipping the bullets to the top of the driving bands in lee Liquid alox, its messy and stinky, but its probably the best lube out there.