Fluting is the removal of material from acylindrical surface in a firearm, usually creating grooves. This is most often thebarrel of arifle, though it may also refer to thecylinder of arevolver or thebolt of abolt action rifle.In contrast to rifle barrels and revolver cylinders, rifle bolts are normallyhelically fluted, though helical fluting is sometimes also applied to rifle barrels.
The main purpose of fluting is to reduce weight, and to a lesser extent increaserigidity for a given total weight or increasesurface area to make the barrels less susceptible foroverheating for a given total weight. However, for a givendiameter, while a fluted barrel may cool more quickly, a non-fluted barrel will be stiffer and be able to absorb a larger amount of total heat at the price of additional total weight.[1][2][3][4]
In the barrelchamber, fluting refers to gas relief flutes/grooves used to ease the extraction of cartridges. They may also come in annular and helical forms.[5] Notable firearms using fluted chambers are theroller-delayed blowbackHeckler & Koch G3 andlever-delayed blowbackFAMAS andAA-52.
Roller or lever-delayed blowback arms require that the bolt starts moving while the bullet is still in the barrel and the spent case is fully pressurized. Fluting the end of the chamber allows combustion gasses to float the neck and front of the cartridge case providing pressure equalization between the front outer surface of the cartridge case and its interior. The roller-delayed blowbackStG 45(M) assault rifle prototypes proved pressure equalization fluting is desirable, since the breech of roller or lever-delayed blowback arms is opened whilst under very high internal cartridge case pressure that presses a spent (bloated) cartridge casing against the chamber walls which can cause significant problems during the cartridge extraction phase. Using traditionally cut (non-fluted) chambers in the StG 45(M) resulted in separated cartridge case heads during testing.