A gas check is not necessarily the be all, end all of leading. It will probably help, but your gun can still lead the bore even with a gas checked bullet.
This is a great article on some of the issues and a means to correct the issues that cause lead bullets not to shoot well or lead the bore
http://www.gunblast.com/FerminGarza-Firelapping.htmI can tell you from personal experience, that if you follow this, you can shoot lead bullets all day long and still only require 2 patches to clean the bore, with zero, zip, nada, leading.
