Not necessarily. The key is the hardness. You can't drive a hard bullet to slow, or a soft bullet to fast. Ie. the bullet needs to be soft enough so that pressure behind it can cause it to expand and plug the bore. Leading is normally caused by hot gases going 'around' the bullet and depositing lead in the bore. When you you try to shoot soft lead to fast, you can get 'skidding' down the barrel as the lands try to spin the bullet. That is why we we talk about about matching bullet hardness to pressure. The bullet must be allowed to obturate (expand). That said, there are other factors that cause leading in a revolver. If the throats are tight is a cause. The throats need to be at least .001 over the bore size. Ruger .45 Colt revolvers are notorious for tight throats. My Ruger .45 Colt revolvers for example have bores of .451. The throats have been reamed to a consistent .4525 as they were all tight. The bullets are all .452 to fit the throats and then 'squeezed' down to .451 by the bore when shot. Another cause of leading is bore constrictions. If they are minor and the bullet hardness is right, leading 'might' not be a problem. If constriction is not minor the bullet is 'sized' down and then gases are allowed to blow by the bullet causing leading. Firelapping is a way to eliminate constrictions. Then there is throat to bore alignment. If slightly off, there can be leading. Setting the forcing cone to 11 degrees can help here. I suppose bore roughness could be a cause too. I haven't run into that one though., but firelapping can help 'smooth' a rough surface too. Just never want to over do it.Leading is caused by trying to drive a cast bullet too fast.