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Posted: 1/9/2006 10:13:19 AM EDT
[Last Edit: 1/9/2006 10:14:56 AM EDT by DK-Prof]
And if so, where does the pressure go?

This relates to my thread abotu a squib yesterday: www.ar15.com/forums/topic.html?b=3&f=118&t=264024

I could have sworn that the rifle cycled, and I noticed the problem when the next round wouldn't chamber proberly (because the squib was only a very short distance into the barrel). But a number of people correctly pointed out that the rifle COULD NOT have cycled if the bullet didn't make it past the gas port (which it obviously didn't), and that I must simply be remembering the sequence of events wrong. Which is entirely likely, since my memory is atrocious . I'm starting to think that perhaps the squib was so weak that it maybe just sounded like a misfire (i.e., just a click of the hammer dropping), making me think that the previous cartridge had failed to eject - and manually operating the handle and trying to chamber the next round. (As an aside, if a cartridge had no powder in it, only the primer - would that be enough to force the bullet a quarter of an inch or so into the bore? and might you not even hear the sound of the primer?)



That may be what happend, but it made me wonder about the following scenario:

What if you had a reduced load, that had enough power to force the bullet ALMOST all the way to the gas port, but not quite - stopping right before the gas port.

Is there no way that the rifle would cycle? I'm guessing no, since the gas pressure is required to unlock the bolt, right? Or is there a way?

If not, does the gas pressure just "sit" in the barrel until the shooter extracts the round by manually pulling the bolt back? Or would it seep out through the rifling, around the bullet, or something?


Just a matter of curiosity, and the operation of the AR - not really important. But I know it's a question that the experts at ar15.com can easilt answer.
Link Posted: 1/9/2006 10:30:07 AM EDT
IMHO, if the bullet does not pass the gas port, the weapn will not cycle. It's not a recoil/gas system, just gas.
As for where the built up gasses went-good question. They were dissipated around the cartridge casing. If it were a perfectly airtight seal there'd be no allowance for expansion, and you'd be extracting all of your brass by hand after every shot.
Link Posted: 1/9/2006 10:39:46 AM EDT
tag
Link Posted: 1/9/2006 10:47:38 AM EDT
No it wont... I bought my M16 last year, got the upper from it right away. Had a buddy check headspace, and we pulled the bbl thinking the index pin was sheared as it wouldnt zero.. Anyways, he put the gas tube back BACKWARDS. Closing off the gas port. I had a single shot upper for a while..Gun would not cycle.. I am assuming the same would happen if the bullet didnt make it past the gas port hole.
Link Posted: 1/9/2006 10:54:14 AM EDT
It couldn't cycle.

The locking lugs on the bolt contain the pressure of the fired round until the bullet passes the gas port, which impinges upon the bolt carrier; as the carrier moves rearward, a cam rotates the bolt , disengaging the locking lugs and allowing the whole assembly to move rearward and start the loading sequence again. Without the gas to move the carrier , everything would remain locked up.

As far as where the pressure goes, if the gas wasn't sufficient to move the bullet out of the barrel, it probably wasn't enough to seal the case to the chamber, and the gas would just leak out after a few moments. As well as leaking around the bullet as well.

As far as just firing a primered round with no powder, the primer has enough oomph to get the bullet at least a few inches down the barrel.
Link Posted: 1/9/2006 12:23:14 PM EDT
I am sure you manually extracted/ejected that squib loaded empty case. Manually extracting, and then chambering a fresh round is what leads up to KABOOOM!
Link Posted: 1/9/2006 12:43:24 PM EDT
The AR-15 is a gas driven action so without high pressure gas BEHIND a bullet passing through the barrel and past the gas port the gun will not cycle. That's why you have to add a Blank Firing Adapter if you shoot blanks...to sustain enough pressure over a sufficient period of time to operate the gas system.

This is different than inertia driven actions that rely on recoil to throw the bolt carrier rearward and function without gas pressure being a factor.

Then of course there are the hybrids which use both...such as the Benelli M4 Super 90 which is an inertia based action with a gas-assist. The cobination of the two makes this a very reliable platform for a wide range of ammo including the less than lethal loads.

Link Posted: 1/9/2006 12:49:18 PM EDT

Originally Posted By Gunbert:
It couldn't cycle.

The locking lugs on the bolt contain the pressure of the fired round until the bullet passes the gas port, which impinges upon the bolt carrier; as the carrier moves rearward, a cam rotates the bolt , disengaging the locking lugs and allowing the whole assembly to move rearward and start the loading sequence again. Without the gas to move the carrier , everything would remain locked up.

As far as where the pressure goes, if the gas wasn't sufficient to move the bullet out of the barrel, it probably wasn't enough to seal the case to the chamber, and the gas would just leak out after a few moments. As well as leaking around the bullet as well.

As far as just firing a primered round with no powder, the primer has enough oomph to get the bullet at least a few inches down the barrel.




Thanks. That's my thinking as well. I must just have assumed that it was a FTE, and manually chambered the next round, and then forgotten I did that in the excitement/shock of realizing there was a squib in there and that I was potentially half an inch away from having blown up my rifle in my face !!

Memory is a funny thing. I seriously could have sworn that it cycled on its own, and that the first sign of anything unusual was the second round failing to chamber completely. Let's hope I am never an eyewitness to a crime and asked to testify !
Link Posted: 1/9/2006 1:03:44 PM EDT
Perhaps there was a second bullet in the case (in-line with the first). Upon ignition, the little bit of powder in the case would have sent the first bullet on its way (unlocking the bolt) until the second bullet sealed the barrel. I don't know if this is possible, but anything goes with PD.
Link Posted: 1/9/2006 1:11:19 PM EDT

Originally Posted By DK-Prof:
Thanks. That's my thinking as well. I must just have assumed that it was a FTE, and manually chambered the next round, and then forgotten I did that in the excitement/shock of realizing there was a squib in there and that I was potentially half an inch away from having blown up my rifle in my face !!



SPORTSKb
Link Posted: 1/9/2006 6:25:24 PM EDT
I assume that a squib has little or no pressure. That's what makes it a squib, thus no action
Link Posted: 1/9/2006 6:42:25 PM EDT
I vote no. About the only way it would cycle would be by direct blowback and that could only happen if the bolt was NOT in battery, in which case the trigger couldn't be tripped and even if the trigger was tripped, the hammer couldn't strike the FP and if the hammer did manage strike the FP with enough force to ignite the round, the result of any full power load going off with the bolt out of battery would be... well, exciting! Just MHO.
Link Posted: 1/9/2006 6:49:22 PM EDT

Originally Posted By imposter:
Perhaps there was a second bullet in the case (in-line with the first). Upon ignition, the little bit of powder in the case would have sent the first bullet on its way (unlocking the bolt) until the second bullet sealed the barrel. I don't know if this is possible, but anything goes with PD.




Now that is an interesting theory.

Not sure it's really possible... But interesting idea no less.
Link Posted: 1/9/2006 7:08:44 PM EDT
Completely cycle, NO.

In your "aside", I do believe that if a cartridge had no powder in it but had a live primer, it MAY have just enough "power" in it to push the bullet an inch or two into the barrel.

At home, I once took out the ball of a 5.56 cartridge, dumped out all the powder and fired the now empty shell from my carbine. I was surprised at how loud the "pop" of the primer was! I was expecting it to be only as loud as a cap gun, but this was louder. But maybe it had something to do with the GP residue still inside the shell.
Link Posted: 1/9/2006 7:13:31 PM EDT
What kind of ammo were you using? If you don't mind me askin.
Link Posted: 1/9/2006 7:16:11 PM EDT
If I may add .... in your second example of a reduced load in a round ...

Assuming that a round / bullet gets lodged inside the barrel BEFORE it reaches the gas port, isn't it possible that the amount of pressure generated by the reduced load be sufficient enough to create a blow-back to push the spent cartridge and the bolt out .... BUT not enough that it will allow the bolt to fully cycle?

just a thought.
Link Posted: 1/9/2006 7:28:18 PM EDT

Originally Posted By JJREA:
What kind of ammo were you using? If you don't mind me askin.



XM193PD. Never using it again - just too much risk.

Sorry - I didn't mention it in this thread, but the entire other thread is pretty much dedicated to that www.ar15.com/forums/topic.html?b=3&f=118&t=264024
Link Posted: 1/9/2006 7:42:05 PM EDT
I vote that the bolt isn't going to unlock unless the bullet passes the gas port .
A rifle cartridge with just a primer and no bullet will be quite loud,extra so as most of us who have tried it did so in the basement reloading room . On the other hand a primer with a bullet blocking the bore,at the shooting range,with hearing protection,and right after the previous round will not make hardly a pop.
Link Posted: 1/9/2006 9:48:23 PM EDT
You guys who keep voting 'I think' are funny. This is a mechanical process, the action of an AR15 cannot, will not move without sufficient gas pressure behind a bullet that has passed the gas port. (excluding gross overpressure for all you "oh yeah, well what if.." guys)


If you look at how the parts of the AR work together it's pretty easy to figure.
Link Posted: 1/9/2006 9:54:55 PM EDT

Originally Posted By eye_spy:
If I may add .... in your second example of a reduced load in a round ...

Assuming that a round / bullet gets lodged inside the barrel BEFORE it reaches the gas port, isn't it possible that the amount of pressure generated by the reduced load be sufficient enough to create a blow-back to push the spent cartridge and the bolt out .... BUT not enough that it will allow the bolt to fully cycle?

just a thought.




Something like that could happen with a direct blowback weapon, such as an Uzi or Mac, but not with an AR. They are very different types of actions... any ammo with sufficient pressure to cause an AR to 'blow back' will destroy the weapon (but this is a moot point because said ammo would have launched the bullet out of the bore, the nullifying the crux of your scenario. But if the bore is obstructed some other way, destructive pressure is very easy to reach ).
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