Montana X-Treme Cleaning Brushes are the toughest nylon brushes in the industry. They are made with very stiff, heavy duty bristles and a solid brass core to assure your firearms will get a superior cleaning with minimal effort on your part. Nylon brushes are necessary when using copper solvents. Standard 8 x 32 thread (.17 and .20 Cal brushes have a 5 x 40 threads).