Technical question on bolt cycling.

[ssmoore]

First off hello to everyone, been lurking for awhile but this is my first post.

What I was wondering was. Does the pressure from the gas tube going into the bolt carrier assy provide all the energy that is used to completely cycle the bolt carrier ? Or is there secondary forces acting upon it thru blow back that helps it overcome the spring pressure? I know this is prolly a stupid question but I was unable to find the amswer in the sticky above.

[K.L. Davis]

The pressure from the gas system is not all there is... in fact, that pressure alone is not enough -- the pressure in the chamber is still forcing the case reward when it unlocks.

[baffle Stack]

I just learned something new.

[K.L. Davis]

I just learned something new.

The pressure alone *can* function the weapon, but that is not the way it was designed -- the infamous "stereo gymnasticator" showed that the gas system pressure may be sufficient to cycle the weapon, but that proper case extraction is a matter of residual chamber pressure. This pressure also is shown in testing to accelerate the bolt velocity.

[ssmoore]

Thanks for the reply. So basically the gas tube pressure unlocks the bolt but most of the force acting apon the bolt carrier assy comes from the residual gases from the chamber. Thats what I thought also but I wondered if the pressure would make the bolt go back to the locked position by overcoming the cam action and gas ring, causing the bolt to be in the locked position as it returns to battery. lol.. I guess I got to think about this some more.

[AR15barrels]

So basically the gas tube pressure unlocks the bolt but most of the force acting apon the bolt carrier assy comes from the residual gases from the chamber.

No, MOST of the function comes from the gas tube.
Only a very small amount of bolt carrier movement is atributed to residual chamber pressure.
Remember that the bullet it out of the front of the barrel well before the case is extracted from the chamber.

The gas from the gas system gets the carrier moving and after a short amount of movement, the excess gas vents out of the exhaust ports on the side of the carrier.
The remainder of the cycle is the MASS of the carrier/buffer going back and compressing the buffer spring and then returning forward.

Tony Rumore demonstrated a pair of M16's saimesed together.
The gas tubes crossing over to the other rifle, much like in the picture above, but with two gas tubes.

Each rifle was running ONLY off of the OTHER rifle's gas impulse.

I have a decent "how it works" post tacked in the troubleshooting section of that other big ar15 website but I will refrain from posting the link here.

[Ross]

Thanks for the reply. So basically the gas tube pressure unlocks the bolt but most of the force acting apon the bolt carrier assy comes from the residual gases from the chamber. Thats what I thought also but I wondered if the pressure would make the bolt go back to the locked position by overcoming the cam action and gas ring, causing the bolt to be in the locked position as it returns to battery. lol.. I guess I got to think about this some more.

The bolt won't be forced back and turn to the locked position because the cam pin is captured in the camway that's built into the upper where the charging handle goes. That's why you have to have the bolt in the forward, or unlocked, position in order to put it in the upper. There's just no way for the bolt to turn once it's unlocked and starts on it's merry way backwards.

Otherwise when the bolt struck the new round from the mag to chamber it, it would rotate to the locked position.

The bolt has to be in the unlocked position to go back out past the locking lugs, and once it's done that there is no way for it to rotate locked until it's returned into position in the barrel extension.

[coldblue]

As stated above, there is some small level of pressure in the bore after unlockng. I have seen this in high speed films of extrator failure with the GI type single extractor springs at high cyclic rate. As viewed from the ejection port, after firing, the bolt can be seen moving rearwards, but without a case. About where the bolt lugs are at the mid-point of the ejection port, here comes the fired cartridge case "jumping" out of the chamber.
This of course led to the next round in the magazine jaming during feeding with this empty case floating around in the breech.
However, the real problem with this scenario was that one would assume this to be a "failure to eject" because one would assume (seeing only the end result) that the fired case in the breeh was extacted from the chamber by the extrator, and this was clearly not the "case."

[AR15barrels]

I have seen this in high speed films of extrator failure with the GI type single extractor springs at high cyclic rate. As viewed from the ejection port, after firing, the bolt can be seen moving rearwards, but without a case. About where the bolt lugs are at the mid-point of the ejection port, here comes the fired cartridge case "jumping" out of the chamber.

Coldblue-

Was this an over-gassed situation causing excessive carrier speed and therefore the extractor jumping the case rim while the case is still obturated in the chamber?

Obviously, when barrel pressure resides enough (bullet leaves bore), there is that little residual pressure in there and that should be enough to pop the case out as seen in the video.

[coldblue]

Coldblue-

Was this an over-gassed situation causing excessive carrier speed and therefore the extractor jumping the case rim while the case is still obturated in the chamber?

I not sure about what "over gassed" is. Except if this carbine had had a souppressor atached to it, but it did not. However, in this case a suppressor woud just added more to the cyclic rate = more extractor lift problems. As it was, it was just "normal" higher than average M4 high cyclic rate and a weak-ass spring/buffer.