AR Sponsor
Posted: 1/26/2010 9:38:52 AM EDT
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I apologize of this has been answered before on this forum. The 30-day search limit sucks, and I couldn't find any thread talking about this subject.
I have also reviewed countless documents on the AR/M16 platform including the Wiki on M16, the Wiki on Rotating bolts, the US Army video on the M16, the write-up posted on this site about the action of the AR/M16. I cannot, for the life of me, find a definitive answer to why the bolt locks into the chamber on these rifles. There are closed bolt guns that don't require the bolt to lock into the chamber (such as the Ruger MKI/MKII/MKIII etc). I know those are rim fired guns. Why can't the bolt shove the round into the chamber, the hammer drops, strikes the primer, etc etc, Why must the bolt lock into the chamber? My guess is that when the bolt locks into the chamber, it prevents gas from the ignited round from seeping directly back into the upper and acting on the BCG (causing a timing issue) before the gas has a chance to reach the tube and come back that way. Am I right? Wrong? Insight please! 
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Quoted: I apologize of this has been answered before on this forum. The 30-day search limit sucks, and I couldn't find any thread talking about this subject. I have also reviewed countless documents on the AR/M16 platform including the Wiki on M16, the Wiki on Rotating bolts, the US Army video on the M16, the write-up posted on this site about the action of the AR/M16. I cannot, for the life of me, find a definitive answer to why the bolt locks into the chamber on these rifles. There are closed bolt guns that don't require the bolt to lock into the chamber (such as the Ruger MKI/MKII/MKIII etc). I know those are rim fired guns. Why can't the bolt shove the round into the chamber, the hammer drops, strikes the primer, etc etc, Why must the bolt lock into the chamber? My guess is that when the bolt locks into the chamber, it prevents gas from the ignited round from seeping directly back into the upper and acting on the BCG (causing a timing issue) before the gas has a chance to reach the tube and come back that way. Am I right? Wrong? Insight please! The bolt does not lock into the chamber. The bolt does not prevent gas from "seeping" out of the chamber. You are comparing a center fire rifle with a rim fire pistol. What major differences are there between rim fire and center fire cartridges? |
| I believe that a straight blowback using all that gas would result in an extreme violence of action, ripping apart the gun unless the bolt was gigantic, look at blowback handguns in heavier calibers (hi-points in 9mm on up) and you will see the slide is a gigantic weighted block. I seem to recall early blowback rifles alternately ripping brass apart and throwing it so forcefully it embedded in wood, reciprocating charging handles that could rip hands apart, and so on. Also lots of explosions. Doesn't take too much force to work the bolt vigorously, so it is locked, and then some of the gas bled off to do it instead. |
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FAQ is your friend http://www.ar15.com/content/page.html?id=535 |
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Ill give a shot at explaining it.
The post aboe about blowback is correct. If the bolt did not lock into the extension the bolt would be alot heavier as well as the the spring weight. When the bolt locks and the round is fired nothing can really move in the action creating a tight lock up which aids accuracy, and only after gas passes through the gas port then back into the bolt carrier does the bolt unlock which means alot of pressure has dropped thus pulling the bolt carrier back and unlocking the bolt. Other than increasing accuracy it also drops the pressure wich makes cycling much less violent. Atleast this is what I understand. |
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There is much more to it than this, but here's a nickel answer.
The rim fire guns you reference are "blow back" actions and they operate at significantly lower chamber pressure than a .223/5.56 round. That low chamber pressure means the bolt can be held in place solely by spring force. Nearly all semi-auto actions for centerfire rounds require that the bolt lock in place long enough for the chamber pressure to decline to safe levels before opening. In reality this is only a few fractions of a second. (Of course there are exceptions/examples of blow back firearms up to the power level of about a .380 acp) The AR-15 platform uses a rotating bolt to achieve lock up. In reality, you could have a blow back operated centerfire rifle fire arm, but the spring pressure would be so heavy you wouldn't be able to move the bolt by hand very easily. If the bolt did not lock in place for this split second all kinds of bad things would happen: severe battering of the fire arm, case rupture, wild variance in velocity and accuracy, just to name a few. |
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Quoted:
The bolt does not lock into the chamber. The bolt does not prevent gas from "seeping" out of the chamber. You are comparing a center fire rifle with a rim fire pistol. What major differences are there between rim fire and center fire cartridges? Rim fire gets dented by the hammer on the rim of the cartidge where are as a centerfire has a primer that is struck by a firing pin. I'm not a complete gun green horn, but perhaps I am just reading the description of the action of a M16 incorrectly? So does the bolt lock into the chamber or not? 2 replies, 2 different answers. This pic seems to show an opening in the chamber (or breech, am I using the wrong terminology?) that would accept the bolt. pictures.second-amendment.org/albums/userpics/10001/upper-feed-ramps.jpg I know Wikipedia is not the best place for information, but this article was the most readily available on rotating bolts: which then rotates the bolt and unlocks it from the breech so that it can be withdrawn in order to extract and eject the spent casing Full article here: http://en.wikipedia.org/wiki/Rotating_bolt So if in fact the bolt doesn't lock into anything (breech/chamber, whats the correct term), what locks on the bolt? Why? Thanks for your patience. |
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Pretty much anything more powerful than .380ACP or 9x18mm Makarov requires a locking mechanism.
The idea is to not allow the case to move backwards until the bullet is well on its way down the barrel and pressures are lower. On a Ruger Mk I, pushing directly against the boltface allows it to move backward. On an AR-15, pushing against the bolt face would not allow it to move backward. So it needs to be done some other way. The AR-15 is gas-operated, so gas is bled out of the barrel and provides the energy to rotate the bolt to allow it to move rearward. |
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Quoted: Quoted: The bolt does not lock into the chamber. The bolt does not prevent gas from "seeping" out of the chamber. You are comparing a center fire rifle with a rim fire pistol. What major differences are there between rim fire and center fire cartridges? Rim fire gets dented by the hammer on the rim of the cartidge where are as a centerfire has a primer that is struck by a firing pin. I'm not a complete gun green horn, but perhaps I am just reading the description of the action of a M16 incorrectly? So does the bolt lock into the chamber or not? 2 replies, 2 different answers. This pic seems to show an opening in the chamber (or breech, am I using the wrong terminology?) that would accept the bolt. pictures.second-amendment.org/albums/userpics/10001/upper-feed-ramps.jpg I know Wikipedia is not the best place for information, but this article was the most readily available on rotating bolts: which then rotates the bolt and unlocks it from the breech so that it can be withdrawn in order to extract and eject the spent casing Full article here: http://en.wikipedia.org/wiki/Rotating_bolt So if in fact the bolt doesn't lock into anything (breech/chamber, whats the correct term), what locks on the bolt? Why? Thanks for your patience. The difference he was getting at is pressure levels. The bolt locks in to the barrel extension. The cartridge itself is what keeps the gasses from going back into the action. The brass expands and seals off the chamber. The breech is the back end of the barrel. The chamber is where the cartridge is seated for firing. Muzzle is the front end of the barrel. Crown is the muzzle end of the rifling. |
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"The bolt does not lock into the chamber"
The bolt does lock a round into the chamber. That is the purpose of the Cam pin! This lock is to insure the explosive force produced by the gun powder is directed through the barrel and not back through the receiver. The gas goes down the barrel until a small portion is redirected to blow back gas tube. The blow back pushes the carrier away from the barrel extension, rotating the cam pin and unlocking the bolt. Sending the BCG free to engage the recoil buffer and then return to chamber the next round. |
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i may be understanding your question wrong, but lemme give this a try. when you look at the end of the bolt you see the lugs. when you look at the barrel extension you see where the lugs slide through when the bolts slides forward. if you take the bolt carrier group out, hold it in your hand, and slide the bolt back and forth, you can see it rotates because of the cam pin.
so when the bolt carrier group is moving forward, the lugs on the bolt slide through the slots in the barrel extension, then the bolt carrier slides forward a little farther, but the bolt itself doesn't move forward. it rotates, and now all the lugs have a piece of metal behind them(locked), instead of open space. as previously posted, those lugs don't seal off the airspace, the brass and boltface does that. so now, the gas coming back down the gas tube pushes the bolt carrier back, which pulls the bolt on the cam pin and rotates it, which unlocks the bolt. ? at least, that is my understanding of how it works. |
whats the pressure of a 5.7? i think its up there and blowback, but like others have said, very heavy internals. check out the bolt on a mac-10, its like a cinderblock flying around inside a tin can.
for the pressures of a 5.56 you need a very heavy spring and a very heavy bolt group. Accuracty would suffer greatly b/c #1 not good lockup #2 the heavy mass flying around would mess up harmonics. its just more practical to have the locking system. |
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Quoted: Respectfully, this is absolutely incorrect. The cam pin does nothing other than partially rotate the bolt and prevent the bolt from being extracted from the carrier."The bolt does not lock into the chamber" The bolt does lock a round into the chamber. That is the purpose of the Cam pin! This lock is to insure the explosive force produced by the gun powder is directed through the barrel and not back through the receiver. |
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Quoted:
Little off topic, but the most powerful handgun I know of to use simple blowback are the Astras in 9mm steyr(400)/ 9mm parabellum(600). Good guns too. Anything more powerful than that? Hi-Point makes a blowback .40 S&W and a 9mm. I think the Astras are 9x23 Largo, not 9x23 Steyr, if that makes a difference. I believe the 5-7 is a delayed blowback of some type. |
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Quoted:
Quoted:
Little off topic, but the most powerful handgun I know of to use simple blowback are the Astras in 9mm steyr(400)/ 9mm parabellum(600). Good guns too. Anything more powerful than that? Hi-Point makes a blowback .40 S&W and a 9mm. I think the Astras are 9x23 Largo, not 9x23 Steyr, if that makes a difference. I believe the 5-7 is a delayed blowback of some type. Yes 9mm Largo for the 400.(I shot steyr in mine, worked fine. Might be the same round). The 600 was 9mm luger tho. |
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Quoted:
Quoted:
Respectfully, this is absolutely incorrect. The cam pin does nothing other than partially rotate the bolt and prevent the bolt from being extracted from the carrier.
"The bolt does not lock into the chamber" The bolt does lock a round into the chamber. That is the purpose of the Cam pin! This lock is to insure the explosive force produced by the gun powder is directed through the barrel and not back through the receiver. Not meaning to argue, but if the cam pin and it's attendant track don't make the bolt rotate, what does? Are we lost in the land of semantics? 
As an aside to the OP, if an AR is assembled WITHOUT it's cam pin, and if everything is lined up just right (or just wrong), and a round is fired, bad stuff happens. Moon |
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"Respectfully, this is absolutely incorrect. The cam pin does nothing other than partially rotate the bolt and prevent the bolt from being extracted from the carrier." Hebrew_Battle_Rifle
Respectfully, Dude you really shouldn't try to post while stoned. The CAM PIN is the most crucial part of the bolt carrier group! Maybe of the entire system. If you studied the mechcanics of your rifle instead of playing mall ninja dry firing at imaginary rag heads you wouldn't make such silly statements! It's a weapon not a toy! When the charging handle is drawn rearward, it moves the BCG rearward and when let loose it chambers a round, the bolt then DOES lock the round into the chamber. The purpose of the Cam pin is to rotate the bolt as it is pushed into the carrier. The rotation of the bolt places the bolt lugs in line with the barrel extension lugs. This locking action is to prevent any movement of the shell casing or the BCG rearward and is to insure that the explosive force produced by the gun powder is directed through the barrel and not back through the receiver. When the gas behind the projectile reaches the gas port a portion of the gas is redirected back to the receiver through the gas tube, in turn pushing against the gas key moving the carrier rearward causing the cam pin to rotate the bolt so that the bolt can disengage (at this point the bolt and barrel extension are pressing against each other, which causes the bolt to pull out enough to allow a cam action rotating the bolt). The ejector which grasped the rim of the shell during battery and holds on to the shell as the BCG moves rearward removing it from the chamber. As soon as the shell clears the receiver, the ejector pin flings the shell casing out the ejection port. At this point the carrier engages and compressess the buffer causing recoil tha sends the BCG forward again loading the next round in the magazine. |
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Quoted: Quoted: Respectfully, this is absolutely incorrect. The cam pin does nothing other than partially rotate the bolt and prevent the bolt from being extracted from the carrier."The bolt does not lock into the chamber" The bolt does lock a round into the chamber. That is the purpose of the Cam pin! This lock is to insure the explosive force produced by the gun powder is directed through the barrel and not back through the receiver. Quoted: "Respectfully, this is absolutely incorrect. The cam pin does nothing other than partially rotate the bolt and prevent the bolt from being extracted from the carrier." Hebrew_Battle_Rifle Respectfully, Dude you really shouldn't try to post while stoned. The CAM PIN is the most crucial part of the bolt carrier group! Maybe of the entire system. If you studied the mechcanics of your rifle instead of playing mall ninja dry firing at imaginary rag heads you wouldn't make such silly statements! It's a weapon not a toy! When the charging handle is drawn rearward, it moves the BCG rearward and when let loose it chambers a round, the bolt then DOES lock the round into the chamber. The purpose of the Cam pin is to rotate the bolt as it is pushed into the carrier. The rotation of the bolt places the bolt lugs in line with the barrel extension lugs. This locking action is to prevent any movement of the shell casing or the BCG rearward and is to insure that the explosive force produced by the gun powder is directed through the barrel and not back through the receiver. When the gas behind the projectile reaches the gas port a portion of the gas is redirected back to the receiver through the gas tube, in turn pushing against the gas key moving the carrier rearward causing the cam pin to rotate the bolt so that the bolt can disengage (at this point the bolt and barrel extension are pressing against each other, which causes the bolt to pull out enough to allow a cam action rotating the bolt). The ejector which grasped the rim of the shell during battery and holds on to the shell as the BCG moves rearward removing it from the chamber. As soon as the shell clears the receiver, the ejector pin flings the shell casing out the ejection port. At this point the carrier engages and compressess the buffer causing recoil tha sends the BCG forward again loading the next round in the magazine. |
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