Bore Tech's Proof-Positive Nylon Bore Brushes are 100% barrel safe and set the benchmark for all other nylon brushes. This is due to the superior construction and engineered features that are incorporated into each and every brush. Unlike phosphor bronze bristled brushes, our premium Proof-Positive Nylon Bore Brushes are strongly recommend when using fast acting copper removing cleaners such as our Eliminator Bore Cleaner or Cu+2 Copper Remover. This is due to the fact that traditional phosphor bronze brushes contain over 90% copper in their composition and will turn patches blue when in contact with most copper removing cleaners. With barrel safe, non-brass cores and threads Bore Tech's Proof-Positive Nylon Bore Brushes are not affected by copper removing cleaners and will eliminate blue patches and the false indication of copper that is associated with traditional phosphor bronze brushes.