A Word to the Wise
As swifter has stated, Flitz is really good for "Pre-break in".......
That said, it's NOT for regular cleaning, especially in a custom barrel.
In fact, I would not use it at ALL in any barrel except a factory tube.
It will ruin the bore with regular use.
It is an abrasive, and will round the corners off the rifling..........
The reason it comes out black, is the fact that it is removing STEEL from your tube.
This is not, powder fouling, or other fouling.
Same goes for JB's, Rem Clean, PSP............same results, over a longer period of time.
So, use judiciously, and on a very irregular basis.
Chemical solvents are the only way to fly to keep your barrels in top condition.........
Hoppes 9, Shooters Choice #7, Butch's Bore Shine,for powder fouling, and some copper removal..and Sweet's 762 for the rough stuff.