I have several .45 Colt Blackhawks with .45 ACP cylinders. On the .45 Colts, I had to ream all the cylinders to open them up and eliminate any internal flaws. On the .45 ACP cylinders, one was ok and 2 others required reaming. All shoot well. The chambers also needed polishing, same as the .45 Colt cylinders. The breach face of the frame also probably needs polishing as well as the step on the face. Check the barrel forcing cone for concentricity, and clean up as necessary. There is also probably a tight spot in the barrel where the barrel screws through the frame. It can be eliminated by firelapping, Taylor throating, or just shooting 5,000 to 10,000 rds.
I have been shooting a pair of the .45 Blackhawks for 4 yrs. as my main match guns in SASS. I have recently switched to .357 Blackhawks for the reduced recoil. When it is time to play, I prefer to play with the .45s