I found this on line, It sounds kinda spookie with zink and lead together??.
""Zinc is a metal with a high lubricity factor. When used as a gas check it will give the bore a mirrow finish after it has "sheradized" the bore. This is a 1-3 mil maximum coating of zinc that reduces friction, and lead fouling thus improving accuracy. You can than push soft lead faster and not see a problem with fouling. A shorter barrel sherardizes with less shots than a long barrel. Well addapted to pistols. These Swage-It Zinc Gas Checks are pure zinc. If you can buy zinc gaschecks for less beware that they are not really pure. Your bore will know that the additional refining cost are wotrth it.""