Burning Question about Bolt Carrier Assembly

[zollen]

I have a burning question about the working of bolt carrier group. I would be much appreciate if you could explain it in detail (hopefully with picture)...

http://image.become.com/imageserver/s6/774448746-150-150-5-0/ar15-bolt/carrier-group-ar15-bolt-only.jpg

How does a bolt carrier lug lock a cartridge in place just before the firing pin ignite the cartridge?


Here are my observations.

1. The teeth on the lug suggests there is a rotating motion during the locking/releasing of a cartridge.
2. The rotating motion must not be guided by inward screw pattern because it could not withstand the extreme pressure for a long period of time, my guess is some form of simple mechanical non-linear rail that guide the rotating motion of the lug.
3. The weight of the BCG is critical for the success of stripping a cartridge from a magazine, and extracting a cartridge after firing.

[Uglyduck]

http://www.youtube.com/watch?v=RKGlthExwRQ&feature=related

The cam pin and the channel it rides in cause the rotating motion

[zollen]

How does the carrier lug lock the cartridge in place? The animation explains the cause of the rotation, but it does not explain how the lug secure the cartridge so that the lug could extract the cartridge after firing.

[Uglyduck]

The extractor claw grabs the case. Disassemble your bolt and you'll see how the extractor works.

[zollen]

The extractor claw grabs the case. Disassemble your bolt and you'll see how the extractor works.

Would you be able to show a picture of a extractor claw? An animation would be even better. I have no AR to work with at the moment...

[Quib]

http://www.youtube.com/watch?v=bJODc5t7SGY

[zollen]

http://www.youtube.com/watch?v=bJODc5t7SGY

This animation explains a lot about the internal working of BCG. I definitely learn a lot from this video.

My Final question, how does an extractor grips a cartridge firmly for extraction? How does the rotation motion locks the cartridge in place? Any diagram/animation to show an extractor in action, would be very nice.

Thanks!

[Quib]

Did you watch the entire video? Take a look again starting at 11:17 minutes.

[zollen]

Did you watch the entire video? Take a look again starting at 11:17 minutes.

It seems the extractor alone is sufficient to grip the cartridge firmly in place, then why the rotating motion? What exactly is locking the bolt behind the cartridge? Is Locking the bolt the exact opposite of extracting a cartridge?

[Uglyduck]

The purpose of a bolts lugs is to lock it in to the receiver extension or the force of a round firing would blow the bolt away from the barrel.

The bolt is like a plug at the end of a tube and when an explosion happens in that tube the energy exerted travels the path of least resistance...out the open end. If the bolt didn't lock, the round would just explode in the chamber and release energy out the chamber into the upper and down the barrel, wreaking havoc along the way.

The extractor grips the bottom of a cartridge and the ejector drives the spent case off the face of the bolt and out the ejection port.

You really need to watch that entire video, it does a good job of explaining the firing sequence.

[Uglyduck]

Quib,
I watched that video after my 2nd post and thought to myself "Quib must've directed this", ironic that you put it up. Its good material for a beginning enthusiast.

[Quib]

Quib,
I watched that video after my 2nd post and thought to myself "Quib must've directed this", ironic that you put it up. Its good material for a beginning enthusiast.

LOL.....1966 was the year I was born. [BD]

[Quib]

It seems the extractor alone is sufficient to grip the cartridge firmly in place, then why the rotating motion? What exactly is locking the bolt behind the cartridge? Is Locking the bolt the exact opposite of extracting a cartridge?

Think of the extractor as fingers, that pull the spent cartridge from the chamber, nothing more.

The bolt rotates, and its lugs interlock with those of the barrel extension. The interlocking of the lugs is what keeps the bolt locked in place during the firing cycle as the gas travels its path through the gas system.

"For every action there is an equal and opposite reaction." If the bolt did not interlock with the barrel extension, the force of the round igniting would blow the bolt backwards as the round propelled forward.

[zollen]

What is barrel extension?

[Uglyduck]

Its what most people think of as the chamber. It threads onto the upper receiver end of a barrel (before the barrel is mated to the upper) and contains the feed ramps and locking lugs that the bolt locks into.

This Link (http://www.ar15barrels.com/making.shtml) shows how a barrel is made and will give you an idea of what one looks like before the extension is mounted.

[Quib]

What is barrel extension?




http://farm6.static.flickr.com/5038/5801808391_e06af4ca74_b.jpg

[Uglyduck]

A thousand words Quib...

[Quib]

A thousand words Quib...


[:)]

[zollen]

So.. the rotating motion of a bolt lug effectively locks itself into the barrel extension? Barrel extension is not part of a one-piece barrel or a upper receiver, is it a separated component?

[Uglyduck]

You got it

Here's another resource to play around with

Interactive AR cutaway (http://www.bushmaster.com/anatomy_bushmaster.asp)

You can control the cutaway to view a specific operation as well as hover over a parts name in a menu and see it highlighted in the cutaway.

[Quib]

You got it

Here's another resource to play around with

Interactive AR cutaway (http://www.bushmaster.com/anatomy_bushmaster.asp)

You can control the cutaway to view a specific operation as well as hover over a parts name in a menu and see it highlighted in the cutaway.


Excellent link, thanks UD.

Zollen, zoom into the interactive video and you can observe the rotation of the bolt, the extractor and ejector. This should answer all your questions.

[oregonshooter]

Is it me or is that Bolt gonna be stuck in the extension for a long time? :)

[zollen]

Is it me or is that Bolt gonna be stuck in the extension for a long time? :)

No, not at all. Here is a barrel extension

http://www.gun-parts-germany.de/images/product_images/info_images/330_0.jpg

As you can see, the bolt carrier lug would only insert into a barrel extension in a certain way. Once a lug is fully inserted, it would rotate itself in order to lock itself inside a barrel extension.