Surprised no one has mentioned the smoothness of the barrel, some barrels need a little conditioning & there are some that seem to lead no matter what. If the bullet fit is correct you can still get leading in a smooth barrel if there's restriction at the barrel threads. If this is the case you will probably have to fire lap that barrel with a few shots to correct the problem.
Also, shooting a few hundred jacketed slugs can help smooth up a rough barrel & then there shouldn't be any leading.
Dick