Quoted By:
Further improvements with the M16A1 from the M16 include things like the forward assist, situated behind the ejection port.
The forward assist is a springloaded plunger which you can use to press forward on the bolt-carrier with, there's a series of serrations cut in the A1 bolt-carrier to allow for the plunger to interact with it. The idea with the forward assist is that since the regular charging handle on the M16 isn't directly connected to the bolt and only lets you pull it rearward, there's no simple way to push the bolt forward aside from letting the spring do its job and push on it, which in some circumstances aren't enough.
The forward assist is actually a rather controversial feature, some will argue that it will only make malfunctions worse, and this isn't untrue, if used at incorrect times, the forward assist will be of no help and can make *some* things worse, it's however my opinion that there are situations where the forward assist will be the right choice of action. A common criticism is that if the bolt is just a nudge from going into battery, there's something in the way and you need to take the gun apart and address that, but fact is that there's more reasons than foreign bodies obstructing the chamber to cause the bolt to shy just out of going into battery, such as dirt and worn springs, or even a damaged magazine.
In some situations, disassembling the gun to inspect the bore isn't something you have the time or opportunity to do, and being able to force the bolt into battery so you can fire right now is more important than the risk of damage to the rifle or further malfunctions (malfunctions which may temporarily be continuously solved by the use of the forward assist).
This makes the forward assist a very contextual feature, but I would very much rather have the ability to manipulate the bolt forward than not being able to at all.