Actually that's a very good question!!!!
Allot of people that shoot lead bullets mistake atimonial wash for leading. Decades ago they started using antimony to harden lead, hence lyman #2 alloy. Antimony is cheaper than tin (lyman #1 alloy was lead/tin). Antimony will leave grey streaks down the length of the bbl, it's called antimonial wash. It hurts nothing, doesn't affect accuracy and is easily removed with a brush.
True leading on the other had is allot harder to get out, but it also tells the user/shooter what's going on with their bullet/bbl/alloy/lube/load combo.
Leading in the 1st" of bbl ='s wrong diameter bullet/too small or too hard of a bullet for the light load being used/no bullet expansion/obturation.
Leading the length of the bbl ='s too soft of an alloy
Leading the last inch of bbl ='s no lube/poor lube/bullet ran out of lube.
A chronograph will easily pick up on what's the best alloy for the load being used/tested/shot.
An interesting link about lead bullets.
Cast bullet alloy's and bullet obturation
I hate to say it but the last time I encountered any leading in a bbl was several years ago with commercial ammo, 22lr ammo of all things. I have a beater k-22 (4-screw) that's had untold #'s of rounds down the tube. Never had a problem before with this pistol and at least 10+ different kinds/brands of ammo. The pistol went from hitting steal @25yds to missing barn doors @ 10 paces in less then 24 shots. I looked down the bbl and shoot my head. This is what came out of the bbl, pushed it out with a rod and a tight patch.
The only other time I've ever experienced leading was when I used super hard bullets in extremely lite loads.
I'm sure other lead bullet users will chime in.