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Forward assisting is the practice of moving the bolt or bolt carrier of a weapon fully forward when the return spring has not done so (or there is a chance that it will not have done so). Some weapons have a dedicated device to allow forward assisting; on others it is simply a procedure performed on the cocking handle.
As a device
The forward assist on a firearm is a button found commonly on AR-15 rifle derivatives, such as the M16 rifle, and is usually located near the bolt closure. When hit, it pushes the bolt carrier forward, ensuring that the bolt is locked. In order to ensure that the extractor is clipped around the rim of the casing, the forward assist is usually struck rather than pushed. It is commonly incorporated into standard loading procedure to prepare a weapon for firing, or to close the bolt when the weapon is excessively dirty.
Another instance where the forward assist can prove useful is when performing a stealth chamber check. Rather than letting the bolt go forward under full spring tension after verifying a round is in the chamber, the bolt can be let forward gently and then the forward assist can be used to fully close the bolt. Doing so will produce a very distinct "click" rather than the loud sound of the bolt slamming forward.
The forward assist had been implemented in 2007 on the MSAR STG-556, an American-made clone of the Austrian Steyr AUG assault rifle, but the usefulness of such device is questionable, since the design is not normally prone to the malfunction that led to the need of the forward assist in other firearms; in fact Microtech Small Arms Research Inc., the manufacturer of the STG-556, has dropped the forward assist on all rifles manufactured since November 2008.
The forward assist may cause the cartridge to become jammed, requiring a cleaning rod to be pushed into the muzzle of the rifle to force the casing back out of the chamber. The high pressure of a round getting hung up when fired could potentially cause an explosion in the rifle. One recommendation is to avoid using the forward assist, and instead dispose of the shell that wouldn't seat properly. The main reason a bolt carrier group (BCG) won't go fully forward is usually an excessive amount of dirt, carbon build-up, etc.
As a procedure
On weapons where the cocking handle is permanently connected to the bolt or bolt carrier, a dedicated device is not necessary as the bolt can be assisted forwards by simply pushing or tapping the cocking handle forwards.
The forward assist is generally not necessary as a standard procedure on any weapon with the exception of the British L85A1. Having realized the frequency with which the weapon jammed when taken outside of the clean environment of the test range, the forward assist was implemented to save the operator the potential danger of aiming the rifle and pulling the trigger and the rifle not going off because the bolt is not fully forward (a safety arrangement called a "safety sear" stopping the hammer from being released and the weapon firing, because of the dangers of firing with the bolt not fully closed).
The design of the L85 makes the forward assist quite awkward as the left supporting arm must come off the hand grip and reach over the top to strike the bolt forward with the left edge of the left hand, much like a "karate chop". A redesign of the L85, known as the A2, alleviated this problem by reducing the number of locking lugs on the bolt and strengthening the recoil spring. However, the "forward-assist" is still taught.