Fluting (firearms)

[1][2][3][4] In the barrel chamber, fluting refers to gas relief flutes/grooves used to ease the extraction of cartridges.

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 blowback StG 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.

G22 of the German Army with a fluted barrel.
Closeup of PGW Timberwolf action with a helically fluted bolt.
Fluted and non-fluted chamber in a blowback firearm
Cutaway model of the chamber with gas relief flutes (left) and roller-delayed action of the G3 battle rifle