View Full Version : Why does carrier go back ?
The question about the gas block reminded me that I have a fundamental question about AR operation too.
Why does the carrier go backwards when gas gets to it? Is it because that's the only way it can go? Or, does some part of the bolt or carrier cause it to move in that direction.
I have wondered about this for many years now.......
militarymoron
09-01-09, 22:12
I have wondered about this for many years now.......
you're kidding right? a 5-second search on google for 'ar-15' leads to the wiki page which explains how it works and answers your question.
http://en.wikipedia.org/wiki/AR-15
Jay Cunningham
09-01-09, 22:14
http://m4carbine.net/showthread.php?t=94
You guys are wrong!
Its MAGIC!!!!! :D
stevenhyde
09-02-09, 01:28
http://m4carbine.net/showthread.php?t=94
That is awesome. ive never seen it before.
Thanks.
I like the bushmaster animation.
http://www.bushmaster.com/anatomy_bushmaster.asp
you're kidding right? a 5-second search on google for 'ar-15' leads to the wiki page which explains how it works and answers your question.
http://en.wikipedia.org/wiki/AR-15
Well, I went to Wiki and here's all it said about the operation of the bolt and carrier, I quote:
"This movement of gas into the bolt carrier forces the bolt and carrier backward in line with the stock of the rifle."
Sorry to be a slow student, but my original question stands. Why does the carrier go back? Is the gas reacting against something and 'pushing' the carrier back? Is the angle of the passage between the gas key and the inside of the carrier pointed toward the back of the carrier? Why does it go back, rather than just sit there? If I blow gas into a cylinder it doesn't necessarily move -- it might blow up, but it doesn't have to move in any particular direction.
Again, sorry to be so slow on the uptake. But, I still don't get it.
Well, in simple terms. The gas pushes on the front, so it moves to the back.
If you push on the front of your car, it moves in reverse.
Do you mean it pushes on the front of the back, inside of the carrier? So the gas key angles the gas backwards as it goes into the carrier? Or, does is the gas aimed at the front of the gas rings, at the back of the bolt?
Well, I went to Wiki and here's all it said about the operation of the bolt and carrier, I quote:
"This movement of gas into the bolt carrier forces the bolt and carrier backward in line with the stock of the rifle."
Sorry to be a slow student, but my original question stands. Why does the carrier go back? Is the gas reacting against something and 'pushing' the carrier back? Is the angle of the passage between the gas key and the inside of the carrier pointed toward the back of the carrier? Why does it go back, rather than just sit there? If I blow gas into a cylinder it doesn't necessarily move -- it might blow up, but it doesn't have to move in any particular direction.
Have you actually done this? The cylinder will move unless something is keeping it from being able to. "Path of least resistance".
Again, sorry to be so slow on the uptake. But, I still don't get it.
Try this:
Take a tall cup or something with a lid. Take a compressed air tank and drill a hole in the lid of the cup and stick the end of the hose from the compressed air tank through the hole in the cup lid and stick the lid on the cup. The lid should be snug fit to the cup. Lay it down sideways. Let a sudden very large burst of compressed air out of the compressed air tank. WHat happens to the cup? Unless the cup is being held by something, it will move.
Your bolt carrier has the gas key on top and the gas tube fits inside this snugly. When the gas comes in (comparatively violently -- this is not you just blowing on it) the bolt carrier starts moving due to the force of the gas hitting it. Because the bolt carrier is inside the upper, and hence constrained, it does not move up or down or sideways, but moves inside the upper receiver -- the path of least resistance is back.
When you hit a pool ball with the cue straight on, does the ball go to the side? Or blow up? No. It moves in lines with direction your cue was going when it hit the ball. Now imagine that there is a pipe that you are hitting the ball into -- the pipe makes sure the ball goes only one direction even if you hit it slightly off.
Same with your bolt carrier.
militarymoron
09-02-09, 08:50
Do you mean it pushes on the front of the back, inside of the carrier? So the gas key angles the gas backwards as it goes into the carrier? Or, does is the gas aimed at the front of the gas rings, at the back of the bolt?
the gas expands into the space between the inside of the carrier and the rear of the bolt. the bolt cannot move forward, but the carrier can move backwards, so it does.
if you tear off one end of a paper straw wrapper and blow into the straw, the wrapper shoots off - same principle.
Steve from ADCO has a good write-up on TOS, I don't know if it's what you're looking for but good info either way.
http://www.ar15.com/forums/topic.html?b=2&f=130&t=165511
spamsammich
09-02-09, 13:05
If that link doesn't help, I don't know what will.
check out youtube and brownells, I believe they both have vides. Brownells is a cut away of a real ar and youtubes fro mthe history channel is a computer model.
Steve from ADCO has a good write-up on TOS, I don't know if it's what you're looking for but good info either way.
http://www.ar15.com/forums/topic.html?b=2&f=130&t=165511
Thanks for the reference. That did it for me. The cut-away upper made it clear. The gas enters a chamber BEHIND the bolt and causes that chamber to "expand" by forcing the carrier to the rear (the only place it can go). As the carrier moves to the rear, the cam pin unlocks the bolt, where after the bolt is free to follow the carrier to the rear.
So, a huge pressure causes a small, but violent movement in the carrier, the inertia of which completes the cycle of auto loading the weapon -- assuming everything is working correctly.
By Jove, I think I've got it!!
Thanks, everyone, for you help and references.
the gas expands into the space between the inside of the carrier and the rear of the bolt. the bolt cannot move forward, but the carrier can move backwards, so it does.
if you tear off one end of a paper straw wrapper and blow into the straw, the wrapper shoots off - same principle.
This.
don't forget that it also requires the introduction of air H20 into the system when the bullet leaves the barrel, thus forcing the gas to follow the path of least resistance which is backwards.
Magic.
I don’t think the surrounding atmosphere has much to do with the cycling and direction of gas travel once the bullet leaves the barrel.
The gas pressure builds up behind the bullet traveling down the barrel. As soon as the bullet passes the barrel gas port, the gasses are redirected through the FSB gas passage, and down the gas tube. By the time the bullet exits the barrel on it’s way to the target, the gasses generated through the burning propellant have already made their way to the carrier. The surrounding atmosphere as the bullet leaves the barrel, has nothing to do with the cycling of the weapon. The gas path is a “closed loop”
http://img.photobucket.com/albums/v489/Metroliner/GasSys.jpg
Steve from ADCO has a good write-up on TOS, I don't know if it's what you're looking for but good info either way.
http://www.ar15.com/forums/topic.html?b=2&f=130&t=165511
I was going to post the same link. It's a good one.
For those fans of piston-operated ARs, this also explains why the DI gun doesn't have carrier tilt.
Magic.Yes, someone else understands! ;)
Thomas M-4
09-03-09, 23:35
I was going to post the same link. It's a good one.
For those fans of piston-operated ARs, this also explains why the DI gun doesn't have carrier tilt.
DI gas everything is nice inline with the bore the way Stoner designed it :p
Getting a little more into the subject based off Steve at ADCOs thread.....When you think about the design differences between gas and piston, why is it exactly that the piston design suffers from carrier tilt?
With Gene Stoner’s original direct gas impingement design, we have gas pressure entering the carrier at the carrier key. This pulse of gas is what initiates the carriers movement reward. The gas pressure, is what sets the cycle of operation into motion.
With a piston driven system we have gas pressure being converted into mechanical energy through a piston and operating rod. At the point on the carrier where the carrier key once sat, we now have the operating rod physically engaging the carrier, mechanically setting the cycle of operation into motion. This point at which the operating rod engages the carrier, is in a way, a pivot point.
As the operating rod strikes the carrier, this mechanical action tips the carrier down and to the rear, tilting the carrier into the receiver extension. The points of contact on the carrier, which stabilize its rearward travel within the upper receiver, are nothing more than the four rails retained from the original design of the carrier.
With the original direct gas impingement designs carrier, we have the four rails which stabilize the carriers rearward travel within the upper receiver. But, does the engagement of the gas tube within the carrier key, act as a fifth point of stabilization? With the direct gas impingement design, does the gas tube/carrier key contact act to stabilize the carriers reward motion, preventing carrier tilt?
I’d like to hear some thoughts on this from the subject experts and industry professionals. :)
Comments I made on Silencer Talk
The AR/M16 Family of Weapons (FOW) was never desgined with a piston in mind. The round aluminum receiver is not the best place to have a carrier impacted by a piston. While we have made piston guns, in both those, and many other manufactuers system we have examined, we have come to the conclusion, that the piston (as it is common accepted, for in truth all AR/M16 FOW have a piston, in the bolt) gas system is not the proper way to run the operating system for a M16FOW.
The receiver is designed for straight line recoil/unlocking forces, not from a top sharp impact. That leads to carrier tilt, receiver wear and early system fatigue.
We found with our rounded lug bolt we can extend the bolt life in excess of 20k rounds. As well as we do not do SBR's shorter than 11.5, for both the reliability increases, as well as bullet velocity increase.
For a shorter weapon system we knew that 5.56mm was not the round, and that resulted in the 6x35 PDW.
The advantages of that is a round (6x35) that is designed specifically for 8-10" barrels and will perform on par with a M4 or Mk18 out to 300m
Plus the design of the system allows for a collapsible stock, while conventional M16FOW architecture (piston or DI) does not. The muscle memory is similar to the M4 (with the addition of a folding stock), so operators can go back and forth with no training issues (as one suffers from when using the 7.5" Sig552, and the M4).
I've personally owned Hk416's and two other piston systems, but sold them, due to the fact I saw no need for the system (and bought a house and it was hard to justify the guns collecting dust)
_________________
the gas expands into the space between the inside of the carrier and the rear of the bolt. the bolt cannot move forward, but the carrier can move backwards, so it does.
if you tear off one end of a paper straw wrapper and blow into the straw, the wrapper shoots off - same principle.
The "...bolt can not move forward..." is not exactly correct. As the gas expands evenly in both (all) directions as it is entering the expansion chamber between the rear of the bolt and the carrier, the bolt is pushed forward to its limit within the barrel extension, which of course is not much, and then stops. The carrier on the other hand is being held forward by the recoil spring which is quickly overcomed by the enery of the expanding gasses. The gas pushing forward on the bolt is a key element of the symmetry of this gas system, as this takes some of the friction load of the bolt lugs during unlocking and aids in extraction of the cartridge case.
Well, I went to Wiki and here's all it said about the operation of the bolt and carrier, I quote:
"This movement of gas into the bolt carrier forces the bolt and carrier backward in line with the stock of the rifle."
Sorry to be a slow student, but my original question stands. Why does the carrier go back? Is the gas reacting against something and 'pushing' the carrier back? Is the angle of the passage between the gas key and the inside of the carrier pointed toward the back of the carrier? Why does it go back, rather than just sit there? If I blow gas into a cylinder it doesn't necessarily move -- it might blow up, but it doesn't have to move in any particular direction.
Again, sorry to be so slow on the uptake. But, I still don't get it.
The pressurized gas flows down the tube, and into the gas key, on the bolt carrier. This much you know. Now we have to determine why it travels backwards as a result. It is a simple matter of physics and geometry.
Once the gas flows into the carrier, it's expansion stops, and pressure builds up even higher. Now, the carrier is fixed into a tube (the upper receiver). Behind the carrier is another tube, as well as the large recoil spring. Gas pressure builds up between the tube and the carrier incredibly quickly, and when the pressure overcomes the resistance of the spring, as well as the friction of the bolt lugs, the carrier is thus hurled back, compressing the spring in the process.
Thus, it moves back because there is only a spring in place to prevent it from moving back in this manner. Once it moves back a bit, the lugs are disengaged as the bolt rotates to allow the carrier to travel, and the entire assembly can travel rearward as designed.
It is actually quite simple in operation, though, it is rather complex in execution.
In actual practice, the gas expands partially within the carrier, to ease this operation, but the principals remain the same. The system is designed to take a violent reciprocating and rotating operation, and dampen it as much as possible. In practice, it works very well.
If you push on something enough, eventually either your arm will break or the object in question will move. In this case, the gas system is essentially pushing on the recoil spring (buffer spring) until it collapses.
militarymoron
09-08-09, 08:19
The "...bolt can not move forward..." is not exactly correct.
it was meant in a simplified context.
Comments I made on Silencer Talk
The AR/M16 Family of Weapons (FOW) was never desgined with a piston in mind. The round aluminum receiver is not the best place to have a carrier impacted by a piston. While we have made piston guns, in both those, and many other manufactuers system we have examined, we have come to the conclusion, that the piston (as it is common accepted, for in truth all AR/M16 FOW have a piston, in the bolt) gas system is not the proper way to run the operating system for a M16FOW.
The receiver is designed for straight line recoil/unlocking forces, not from a top sharp impact. That leads to carrier tilt, receiver wear and early system fatigue.
We found with our rounded lug bolt we can extend the bolt life in excess of 20k rounds. As well as we do not do SBR's shorter than 11.5, for both the reliability increases, as well as bullet velocity increase.
For a shorter weapon system we knew that 5.56mm was not the round, and that resulted in the 6x35 PDW.
The advantages of that is a round (6x35) that is designed specifically for 8-10" barrels and will perform on par with a M4 or Mk18 out to 300m
Plus the design of the system allows for a collapsible stock, while conventional M16FOW architecture (piston or DI) does not. The muscle memory is similar to the M4 (with the addition of a folding stock), so operators can go back and forth with no training issues (as one suffers from when using the 7.5" Sig552, and the M4).
I've personally owned Hk416's and two other piston systems, but sold them, due to the fact I saw no need for the system (and bought a house and it was hard to justify the guns collecting dust)
_________________
I think the two big factors in the PDW system being successful is cost of the system and the availability of ammunition. In my opinion, the cost has to come down and ammunition has to be produced in large enough quanities so when ordered people don't have to wait a year and get 20 round from a 1K order.(not referring to anything with KAC ref ammo)
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