These are high quality, polyurethane tank o-rings for use with paintball CO2, HPA/Nitro tanks. Polyurethane as a material is much more abrasion resistant than black buna-nitrile which is often sold as tank o-rings. Buna o-rings nick and tear when screwed into your marker's ASA, causing leaks from the tank. Polyurethane lessens the need to replace this o-ring.