A barrel’s rifling consists of a series of spiral lands and grooves running down the length of the bore. Lands are the raised areas between the grooves. The ultimate purpose of rifling is to provide a fired bullet the proper twist or spinning motion for greater accuracy. Barrels can be manufactured with any number of lands and grooves in various twist patterns. A few of the more common rifling patterns are 4-groove right hand twist, 5-groove right hand twist, 6-groove right or left hand twist, and 8-groove right hand twist.
- Button Rifling: The most common method of rifling is known as button rifling. Button rifling uses a different approach to forming the grooves in the barrel. A “button” 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 hardens and polishes the inside of the barrel at the same time.
- Broach Rifling: The modern broach method of rifling uses a hardened steel rod with several cutting rings spaced down the rod. Broaches can be over 16 inches long and because they have several cutting rings, they are referred to as “gang broaches”. Each successive cutting ring is slightly larger in diameter and when the last ring on the broach passes down the barrel the desired groove depth is obtained. The cutting rings have gaps evenly spaced around them to allow for the lands. The rod is twisted as it is pushed or pulled through the barrel, which forms the spiral rifling pattern.
- Cut-Rifling: This is the oldest and most accurate method of rifling. Cut rifling creates the spiral grooves in the barrel by removing steel with the use of a “hook cutter”. The cutter rests in the cutter box, which is a hardened steel cylinder made so it will just fit the reamed barrel blank, and which also contains the cutter raising mechanism. The cutter box is mounted on a long steel tube, through which coolant oil is pumped, and which pulls the cutter box through the barrel to cut the groove. As it is pulled through it is also rotated at a predetermined rate to give the necessary rifling twist. A passing cut is made down each groove sequentially and each cut removes only about one ten thousandth of an inch from the groove depth. After each passing cut the barrel is indexed around so that the next groove is presented for its passing cut. After each index cycle the cutter is raised incrementally to cut a ten thousandth deeper on the next cycle. This process continues until the desired groove diameter is reached. It takes upwards of an hour to finish rifling a barrel by this method. The hydraulic machines used in many custom barrel shops are invariably Pratt & Whitney.