Slim I'm gonna disagree with ya, I bets the bullets are too hard, is this revolvers, if so that is most likely the problem. Just make a tight patch with 0000 steel wool, that is what the serious cast bullet shooters most often use, ya know the shutezen and the Cast bullet As. bench guys, won't hurt a thing, just use a little Hoppe's on it.
Jacketed bullets leave behind copper fouling, if the gun has ever had jacketed bullets shot in it, get the lead out and then get every trace of the copper out. Use a good copper solvent, wet the bore and let it sit an hour, if there is any trace of blue on the patch do it agaiin and again till there ain't no blue. Lead likes to stick to copper fouling.
Most store bought bullets are too hard for maximum performance in the light CAS type loads in revolvers, fact is they are too hard for almost any revolver use. The reason is though the alloy most bullet makers use is good for casting, not a lot of rejects.
What are you shootin' that is leadin' sounds like it is a bad problem if yer cussin' it. Much easier to prevent it than to clean it.