Lightening holes are holes found in various firearm components used to make the weapon lighter in weight as well as increase the ruggedness and durability. They are usually made by drilling holes, pressed stamping or machining and can also save strategic materials and cost during wartime production.
The edges of the holes may also be flanged to increase the rigidity and strength of the component. The holes can be circular, triangular, ovals, or rectangles and should have rounded edges, but they should never have sharp corners to avoid the risk of stress risers and they must not be too close to the edge of a structural component.
Lightening holes are also found on handguards and air cooling jackets on firearms. They can also be used as vent holes, sling swivels, hold shotgun shells/rounds and for picatinny rails etc. During WW2, lightening holes were used on aircraft machine guns to increase the rate of fire and performance by machining out the bolt. Various tripods and vehicle/aircraft mounts use lightening holes to reduce weight as well as stability for the weapon when firing.