The bipod is a set of two legs, angled at roughly 45 degrees, and attatched to the undershroud on a scoped rifle or machine gun.
The idea behind the bipod is to provide a stable firing platform for either a long distance highly powered weapon, or a heavy, fast firing weapon, reducing the strain on the gunner's support arm.
When placed on a heavy machine gun, this avoids the gunner being shaken to pieces whilst attempting to support a rapid firing, usually heavy caliber weapon. (There are mulitple stories from books based from WWII where soldiers have, in the confusion of an attack and/or withdraw, have run along firing these heavy weapons from the hip, often hitting their own soldiers, or dropping the weapon completly. this has been known to lock the firing mechanism and sending a big heavy rabid firing beast spinning around on the ground hitting anything in the way.)
On a machine gun, the bipod is fitted to the actual furniture of the weapon, not the bipod. This is due to many weapons having interchangable barrels, ( a hot barrel decreases the accuracy of the weapon, as the heat expands the diamter, rendering the rifling useless, and could also cause the round to rotate the wrong way, blocking the barrel)
In terms of a scoped rifle, it gives the rifleman a firm anchor at the end of a (typically) long barrelled weapon. This would avoiding the muzzle from moving around, allowing a hit on a target (some modern rifles fired by trained snipers are capable of scoring hits from over 1km away, some reportedly head-shots).
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