Tetra Prosmith Nylon Bore Brushes will not harm your barrel. Tightly wound precision brass core brushes effectively remove fouling from gun barrels. Industry standard 8-32 male thread (5-40 for.17 Cal. &.204 Cal.)-5/16-27 male thread (12,20,28 & 410 Gauge)-Close tolerance specifications for perfect bore fit-Will not harm or scratch steel-Durable and reusable.