Button rifling uses a different approach to forming the grooves in the barrel. A button as seen below is a very hard steel plug that is forced down an unrifled barrel. The grooves are then formed in the barrel under very high pressure. The pressure created to form the rifling in the barrel hardens and polishes the inside of the barrel. The newest method of rifling is accomplished through a process called hammer forging.
Hammer forging produces a type of rifling called polygonal rifling .
A hardened steel mandrel is produced with the shape of the rifling formed on its outer surface. The mandrel is inserted into a barrel blank and the outer surface of the barrel is machine hammered. The hammering forces the barrel material down against the mandrel and the inner surface of the barrel takes on the shape of the mandrel. The mandrel is then removed from the barrel and the outer surface of the barrel is cleaned up. Just as in the other types of rifling, polygonal rifling can have different patterns.