For many years BSA has been making gun barrels of the highest quality, today the company enjoys an enviable reputation amongst its peers and supplies barrels to numerous other gunmakers. The rifling in air rifle barrels is used to spin the pellet as it leaves the barrel and to give a gyroscopic effect to it’s flight that considerably improves its accuracy, particularly over longer distances. BSA uses 12 groove rifling with a twist of 1 in 17 inches.
Special steel is used for BSA barrels which work hardens when it is cold swaged to enhance the wearing qualities of the rifling. The barrels are produced from solid bar, this is deep hole drilled on specialist deep hole drilling machines, a method of manufacture that ensures that the bore is straight and has a superior surface finish ready for swaging. Many other air rifle manufacturers use tube to produce their barrels, however the process of “drawing” the tube can result in a “skin” forming on the surface of the bore which can result in a “flaking” condition during later machining operations.
After deep hole drilling the barrels are rifled on a cold swaging machine where the barrel blank is passed between 6 rotating hammers, each weighing approximately 20lbs, over a specially machined tapered mandrel. To swage a single blank takes over 35,000 hammer blows. The action of this cold swaging increases the length of the barrel by 15%; work hardens the surface of the rifling, refines the grain structure of the steel, and produces a mirror like finish.
The tapered mandrels are made from specially formulated tool steel, which is manufactured in small batches under laboratory-controlled conditions. It is then CNC machined to the most exacting tolerances to produce the finished mandrels, each of which costs well over £1,000 but is capable of producing several thousand barrels.
The form of the rifling is reproduced as a mirror image on the mandrel. The swaging machine is able to accurately position the mandrel between the centre line of the hammers, and by setting its position up or down its tapered length, the size of the hammered bore of the rifling can be adjusted to very fine limits.
A further big advantage of the cold swaging process is that it can be used to produce a “natural” choke at one end of the barrel. The normal method of choking is to finish the barrel and then squeeze the outside diameter to reduce the bore size, but in some circumstances this can distort the pattern of the rifling. However with the BSA method the rifling form is reproduced perfectly from the impression on the mandrel, with the bore gradually decreasing very slightly in diameter to give a perfect choke with considerable benefits in accuracy.