Chrome lining is a treatment applied to the inside of SIONICS barrels. Chrome lining is essentially an alternative to a Melonite/nitride finish. During this process, chrome is applied to the lands and grooves of the barrel’s rifling, as well as the chamber. In order to ensure accuracy with this extra material, chrome-lined barrels are typically bored out more than the final internal diameter. The tolerances are made up by adding the chrome to the inside of the bore.