In today's post, we will study another mode of firing: burst mode. Burst mode is an intermediate between semi-automatic and full automatic firing modes. In semi-automatic mode, the weapon will fire one round per trigger pull and the user has to release and pull the trigger again to fire the next shot. In full automatic mode, the weapon will continue to fire automatically as long as the trigger is held down and there is ammunition available in the magazine. While full automatic firing provides a lot more firepower than firing in semi-automatic mode, it also tends to waste a lot more ammunition, especially if soldiers are inexperienced and hold down on the trigger for longer than necessary. The recoil from firing in full automatic mode also leads to inaccuracies. Burst mode provides a compromise between these two firing modes. When a firearm selector is set to fire in burst mode, it will fire up to a set number of rounds (usually 2 or 3 rounds) per trigger pull. After that, the user has to release and pull the trigger again to fire the next set of rounds and so on.
In Vietnam, the US military found that new soldiers often ran out of ammunition in combat, because they had set their M16 rifles in full automatic mode and shot their entire supply of ammunition in a few seconds (and often without hitting their targets). Therefore, they requested that the M16A2 model remove the full automatic mode option and implement a burst mode instead. Their studies showed that a three-round burst provides the best balance between firepower, accuracy and conservation of ammunition. This is why the M16A2 and M16A4 models and the M4 carbine models have a three-round burst mode.
A person named "Stealth the Unknown" has prepared a great video showing how these different firing modes were implemented on the M16 family of rifles:
In the case of M16 models, the burst mode is implemented by a rotating cam. The same video also describes how the mechanism works for semi-automatic and full automatic modes.
The same author also prepared a second video answering some follow-up questions about this mechanism.
For instance, in a M16, if the user releases the trigger before a three round burst is complete, then the next trigger pull will only fire 1 or 2 rounds. This is because the M16's cam mechanism does not reset when the trigger is released. In some other firearms, the mechanism resets every time the trigger is released and therefore the next trigger pull will fire the full number of rounds. The author of the video also goes into an interesting theoretical design where he designed a selector with multiple burst firing modes as well as a semi-automatic and full automatic modes.