Leading is caused by shooting an undersized bullet. It's often not the base of the bullet that causes the leading but the gas escaping around the circumference of the undersize bullet as it travels down the bore of the barrel.
Disregard the crap you read by no nothing gun writers about using a bullet that is bore size or 1 thousandth over, rather shoot the biggest oversize lead bullet you can lay your hands on assuming that the cartridge will still fit into the chamber. No barrel is of uniform diameter, they do not exist, as all barrels have loose and tight spots, even hand lapped custom barrels. Some factory barrels are so bad you can even feel the loose and tight spots by pushing an oversize cotton dry patch down the bore with your cleaning rod.
Years ago I bought a .44 magnum and got leading so bad you could not see the rifling after a few shots. I was lucky my revolver did not blow sky high. I tried everything until I observed that factory lead ammo which is almost pure soft lead with only about 3% antimony added to it gave me almost zero leading. So what was wrong with my hardcast handloads? It was the fact that the bullets were slightly undersize in relation to the bore diameter and short of having an expensive custom made oversize bullet mold made I quit using fast burning pistol powder like Bullseye and Unique and went to a slower burning powder which included both 2400 and 4227 which gave me cooler burning because it does not burn so quickly and I was able to use more powder creating enough pressure to obturate the lead bullet. The result with my original hard cast bullets (still the same diameter) resulted in very little leading,. In other words the hard cast bullet was expanding in diameter on firing and the bigger powder charge and cooler burning powder did the trick.
I also noticed that when shooting cast bullets with gas checks which prevented the base from being distorted the gas check did not prevent leading in the .44 magnum at all because the bullet was undersize and the powder used was fast burning Bullseye.
In rifles I got no increase in leading by not using gas checks when the bullet was oversize in relation to the rifle's bore but the gas check did give me better accuracy because it prevented the base of the lead bullet from being distorted by burning powder. In pistols with their lower powder charges as compared to rifles gas checks were never necessary and I did try them in the .44 magnum when I was having so much trouble with leading and the gas checks did nothing to prevent the leading. As far as accuracy I never noticed any increase in accuracy when using gas checks which was quite the opposite of using them in rifle bullets.
I forgot to mention I generally size my cast bullets 3 thousandths oversize of bore.
By the way I shot my first cast lead bullet handload in 1968 out of a Browning High Power because even back then jacketed bullets were so much more expensive to use than a cast lead bullet. And a properly made cast bullet will shoot every bit as accurate as a jacketed bullet. If they did not NRA Bullseye competition shooters would never have used them for decades and long before any of us now living were even born.
