I intend to post this so please rip it to shreds... improvements, additions, corrections of my misconceptions, or just general comments.

I am intending to post this on my website and I have tried to make it as accurate as my current knowledge allows. diagram references are according to the exploded diagrams listed here on AR15.com


"BC = bolt carrier assembly
B = bolt assembly
LR = lower receiver
UR = upper receiver
URA = upper receiver assembly

We will begin with a round properly chambered. The user pulls the trigger (LR29), releasing the hammer (LR26). The hammer flies forward, striking the firing pin (BC2). The firing pin sits inside of a shaft that runs the entire length of the bolt itself. The pin strikes the primer on the round, igniting the powder and sending the bullet down the barrel. The brass casing expands slightly due to the pressure, sealing it against the walls of the chamber. At this point, the bolt is locked in place and very little pressure is able to escape rearward – most of the hot gasses are now pushing the bullet.

When the bullet passes the gas block, some of the gasses are able to enter the gas tube (UR3). This tube bleeds off some of the pressure and sends it back down to the bolt carrier key (BC5). The pressure begins to force the bold carrier rearward. But wait! The bolt is locked. This is where the bolt cam pin (BC3) comes in. The way the cam pin is situated, as the bolt carrier moves rearward, it forces the bolt to rotate and unlock (This behavior can be seen by removing the bolt carrier and manually pushing the bolt into the locked and unlocked position – notice the movement of the cam pin). When the cam pin reaches its stop the bolt is fully unlocked, and the bolt is forced to move rearward with the bolt carrier. At the same time, the extractor (B2) is gripping a small rim on the spent brass, causing the brass to come with the bolt. The bolt carrier is now pulling the bolt along, which is pulling the brass along.

While at rest, a notch in the ejection port cover (URA20) sits inside an indentation on the bolt carrier. However now that the bolt is moving rearward, the indentation slope moves past the notch, forcing it out. That’s all the Ejection Port Helical Spring (URA19) needs and it begins to force the port cover open. If the Port cover was already open, these steps are skipped.

The ejector (B5) then begins to push on the spent brass, from the side of the bolt opposite that of the extractor. This all happens in one fluid motion resulting in the spent brass being flug out the ejection port.

The bolt carrier group continues to move rearward and impacts the buffer assembly (LR13). This spring absorbs the impact of the bolt carrier group. At this point, the firing phase is over and the recovery phase begins. The spring in the buffer assembly releases the absorbed energy (minus heat losses) pushing the bolt carrier group forward. As the bolt carrier group moves forward, it pushes a fresh round forward in front of it. (The magazine spring has been exerting upward pressure on the rounds, and when the rearward moving bolt carrier group uncovered the magwell, a round moved upward just enough so that the bolt carrier group moving forward would push it.) The round slides upward and is guided into place by the movement of the bolt carrier group.

The bolt passes the locking lugs. At that moment, the bolt presses against the new round and seats it in the chamber. At the same time, the bolt carrier is still carrying forward momentum. This exerts pressure on the cam pin, causing the bolt to rotate back into the locked position. This locks the bolt and bolt carrier assembly into place, and the rifle is again ready to fire. The firing pin may still carry momentum and lightly impact the primer (resulting in a visible dimple.) This is expected and normal; the primer in good ammo is formulated to resist “slam-firing”. "


Let the merciless slaughter words begin.

Thanks!

Forgot one kind of important part.  As the gas flows into the bolt carrier it also pushes the bolt forward relieving some of the stress on the locking lugs.  The expanding gases fill a chamber that is sealed by the gas rings on the bolt and the bolt carrier.  As the bolt is pushed out of the carrier the key rotates the bolt.  As the bolt carrier moves rear ward and the bolt stays stationary the gas rings pass by some venting ports in the bolt carrier and excess gas is now vented through the ejection port.  

This is the "direct impingement" part of the cycle.

Feel free to fez this up as you see fit or if you have any questions ask away.

Starting condition: Cartridge locked in chamber, bolt locked into barrel extension, firing pin at rest near, or against, the primer, hammer is cocked onto trigger nose.

1) The trigger is pulled releasing hammer.
2) The hammer rotates forward and impacts the rear of the firing pin.
3) The firing pin impacts the cartridge primer.
4) The powder in cartridge ignites and the expanding propellant gases push the bullet into the barrel.
5) The bullet passes the gas port.
6) Propellant gases enter the gas port and begin moving into the gas tube.
7) Propellant gases are directed along the gas tube into the bolt carrier through the carrier key.
8) Gas moves inside the bolt carrier through the hole in its top that is covered by the bolt carrier key and is captured between the bolt rings and the tail of the bolt.
9) The bullet leave the bore.
10) The gas pressure causes the carrier to move backwards against the action spring beginning the recoil stroke.
11) As the carrier moves backwards, the bolt cam pin begins to move forward along the curved cam pin track on the top of the bolt carrier.
12) The movement of the bolt cam pin along its curved track causes the bolt to rotate and unlock its lugs from those of the barrel extension.
13) As the cam pin rotates its head moves out of the clearance cut on the left side of the upper receiver and towards its guide track.
14) As the bolt reaches its fully unlocked position the head of the cam pin engages the guide track in the upper receiver; this holds the bolt in its most forward (-unlocked) position.
15) When the bolt reaches its fully unlocked position the bolt rings clear the two vent holes in the right side of the bolt carrier body and excess propellant gas is vented through them.
16) Simultaneously, the seal between the carrier key and the end of the gas tube is broken causing the gas pressure to drop in the gas tube.
17) The extractor is hooked into the extractor groove of the empty cartridge and holds the empty cartridge to the face of the rearward moving bolt.
18) As the bottom of the carrier moves over the top of the hammer it pushes the hammer down and the disconnector latches onto the middle hammer hook.
19) The tail of the trigger hits the bottom of the selector and prevents the disconnector from rotating too far forward thus interfering with the operation of the hammer.
20) The ejector presses against the left side of the rear of the cartridge case causing the cartridge case to drag along the right side of the chamber.
21) As the bolt carrier assembly continues moving to the rear into the lower receiver extension the front of the empty cartridge case clears the ejection port.
22) The ejector spring expands, driving the ejector forward, pushing the cartridge case out of the ejection port.
23) The case pivots on the extractor hook and leaves the ejection port.
24) When the action spring is fully compressed, it pushes the bolt carrier assembly forward; this begins the battery stroke.
25) The bottom lugs of the forward moving bolt push the next round out of the magazine and toward the feed ramps on the barrel extension.
26) The cartridge is pressed into the chamber by the forward moving bolt and the shoulder of the cartridge striking the shoulder of the chamber stops the forward motion of the bolt.
27) The face of the bolt lugs strike the rear of the barrel inside the barrel extension ensuring that the bolt stops moving forward.
28) The carrier continues moving forward under pressure from the compressed action spring.
29) The head of the cam pin enters the recess on the left side of the upper receiver and disengages from the guide channel that has kept it locked in the forward position.
30) The moving carrier forces the bolt cam pin to move backwards along the cam slot in the bolt carrier causing the bolt to rotate into its most rearward (-locked) position.
31) The rotating bolt locks its lugs into those of the barrel extension.
32) The trigger is released allowing the hammer to slip off the disconnector and onto the nose of the trigger.
33) The rifle is ready for the next shot.


I didn't have time to double check this, I have checked it previously tho. I had created it for a class then remembered the much better explained and beautifully illustrated version in the FM 23-9.

The rear of the bolt carrier is always in contact with the face of the buffer when the rifle is assembled. If it's not then something is wrong.

[url]http://66.155.8.108/streaming/streamingMain.htm[/url]

Tweak resurrecting old threads again!

You also forgot to mention the rearward movement of the carrier cocking the hammer and the hammer being captured by the disconnector and the hammer is transfered back to the trigger when the trigger is released.

Alternatively you could describe how the auto sear works as the carrier moves back into battery if describing full auto.