Hey Jim,
Yes, I've noticed accuracy fall-off on all Improved cartridges from carbon build-up in the front of the chamber. They all shorten when fireforming and it is basically the nature of the beast.
Obviously, some powders foul more than others and it is best to avoid known foulers. For instance, WW 748 tends to be dirty, so while I often use it in a .223 Remington, I never use it in my .223 Ackley rifles.
Having said that, I assume you are using the data that I published in The VARMINT HUNTER Magazine. I am quite (painfully) aware of the carbon situation and tried to avoid problem powders in the load data.
The carbon is easily removed with a good bore brush and Shooter's Choice or, even better, Shooter's Choice Black Powder Gel. Actually, I use the Black Powder Gel in place of the liquid normally anyway because it is much more user-friendly. I'm also personally convinced that the Black Powder Gel does a better job of break-in and general cleaning.
I really love the .260 Ackley and I'm delighted that you are having fine accuracy with it. please keep me up to speed on your project, my friend.
My very best, Steve