The Nylon Brushes are constructed of a material that is unaffected by most modern powder and copper solvents. This resistance is a bennefit over bronze brushes. The bristles are designed to be very ridged providing maximum bore scrubbing performance. Molded hinge top storage box included. Threads are industry standard: 8-32 on .22 cal through .45 cal. The .17 cal has 5-40 threads.