I use it for break-in of all barrels including custom.
Reamer born edges form in any barrel, and that's what I smooth/settle out.
FF will reduce copper fouling greatly in barrels bad for this. Unless used too much... Don't do this, or fouling will return due to polishing.
At that point the barrel would have to be lapped to roughen it back up.
Basically, too smooth is as bad or worse than too rough(for copper).
Custom barrelmakers lap to a perfect finish for minimal copper. But 10 FF bullets doesn't hurt this(doesn't help either). And, the FF bullets do quickly break down edges left from chambering.
Every now & then I load one or two FF bullets just to square up retreating throat a bit.
My suspicions were that this would be helpful,but there are a lot of folks selling snake oil out there.I,ve decided to halt my load development until I can smooth up this bore some.It,s fouling up to the point where I think its affecting pressures and its a pain to clean,Takes too long to do it right in the field.I thank all of You for your wisdom and generosity with it.GRPB