The Black Diamond being the design it is, is messy. I have one. And for years I cursed the blow back when shooting a 209 ignition system. But since I shoot Goex, Pyrodex, or Triple Seven loose powder out of it, I changed back to a #11 percussion cap system. And I thank the forum members that suggested I try that. Even an old dog can learn new tricks. You can't believe how much cleaner the rifle is with this set up.
As for accuracy, the Black Diamond I have never let me down on the accuracy department. Even with the bad blow back.. it was deadly accurate.
With a simple 4x scope on it, I was shooting white paper plates at 150 yards with ease, once I learned the drop. Now I am not saying it was a 3 inch group at that distance. But I could keep them in the center area of the plate which means I could have shot a deer with ease at that distance.
But if you get a chance, try that #11 nipple. you might find a whole new respect for the rifle.