If the bullet is to hard and not a great fit at low pressure you can get blow by , this happens more with bevel base and gas checked bullets in some guns ( and it depends on the gun ) plain and hollow base bullets are not as likely to show signs of blow by . The groove that holds the lube seems to work better if it has a square bottom .the bullet reacts in a way that forces the lube out of the grove better . a longer bodied bullet stands a better chance of entering the barrel in a straight line better than a short light bullet in a revolver !
barrel leading can come from different sources , most is from bullet skip where the bullet skips before engaging the rifling due to poor fit of bullet , bbl or bbl and cyl being out of alignment . bullet hardness can be used to correct
some of the problems with regard to hardness either harder or softer . If you are just getting started a good bullet is the Kieth style for revolvers it addresses the bottom of the lube groove , bullet length to bore size and in hardness if obtained from a reputable company ! its not the end all but a place to start ! the cowboy bullets seem to have bevel bases , they load easy but seem to yaw sometimes at distance , cowboy matches are relative short range so it doesn't matter and may not to you unless ya want to shoot at distance !
check out a Lyman reloading manual , they have a mix of 9 lbs of wheel weights and 1 lb 50/50 solder if i remember correctly , it was used as a standard ! any way it is a good place to start as is a R E Lee manual
