How a bolt carrier group works
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Firing
Let’s start with the trigger pull and work our way from there. When the trigger is pulled the hammer falls and hits the firing pin that is in the Bolt carrier group pushing it forward violently enough to ignite the primer of the bullet.
Unlocking
The bullet starts going forward due to expanding gas created by the gun powder. There is a small hole in the top of the barrel for some of the gas to be redirected through the gas tube back to the bolt carrier group. The force of the gas slams into the gas key making the bolt begin to launch backwards. Once the bolt carrier group starts to move backwards the bolt begins to turn which unlocks it from the barrel extension. This is done by the cam pin which is designed to help tun the bolt as the carrier moves rearward.
Extraction and Ejection
After the bolt unlocks it begins to move backward with the carrier. There is an extractor that grabs the rear of the case and as the BCG moves back the empty case is yanked from the camber. The ejector that is in the bolt face opposite of the ejector pushes the casing on one side forceful enough to fling the brass out the ejection port once it clears the chamber.
Cocking the hammer
While the Bolt carrier group is flying rearward it is doing all the things just mentioned but it is also re cocking the hammer. The hammer after firing is rested against the firing pin inside the BCG and as it moves backward it brings the hammer back down and locks it back in place getting ready for the next trigger pull.
Feeding
I feel like this is a good time to kind of explain buffer systems. There is a metal piece that sits inside of a big spring that absorbs the energy of the BCG on the way rearward so there isn’t just a hunk of metal flying toward your shoulder. Once the BCG comes back as far as it can the spring and buffer push it forward. The magazine properly installed will have moved the next round to the top causing it to stick up in the way of the returning BCG. The BCG will push the round forward with it and cause it to feed into the chamber by way of the feed ramp.
Locking
Finally, the bolt carrier group will chamber the round and return to the barrel extension. As it slams back home into the extension the bolt will rotate again and lock into place. Once the bolt locks in it is ready to fire and repeat the process.
Foreword
So now that you know how it works there are some very important things, I’d like to explain from an engineering perspective. There are a few critical parts in the bolt carrier group.
Bolt
The bolt that controls the explosion needs to be of the best quality for durability. There are two types of metal typically used. That is 9310 and carpenter 158.
Firing pin
The firing pin is something else that handles a lot of stress and should be coated and free of deformities. Most are stainless steel and coated in nitride or chrome.
Carrier
On the carrier the only thing that touches the inside of the upper thus creating friction are the skids which there are two on top and two on bottom keeping it in the right place and guiding it though the process. The gold standard metal is 8620 and there are a lot of different coatings people use. We use ArmorLube.
Cam Pin
There are two parts that connect the carrier to the bolt. They are the gas rings and the cam pin. The cam pin is what locks and unlocks the bolt and is held in place by the firing pin. This should be coated in a very durable coating and the metal should be properly heat treated.
Gas rings
There are three gas rings that create a seal between the carrier and the bolt. These are subject to a lot of friction and a lot of heat. These are some of the highest wear items on a BCG. That’s why we do something different than everyone else. We coat them in a coating that is very hard and has very high lubricity, so they last longer and make locking and unlocking a much more reliable process. I’ll dive more into that in the next article.
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