View Full Version : Techie Question
Forgive me if this has been answered here can't fine it.
Does the BCG unlock and start to move rearward (with less force) before the bullet passes the gas port and then after the bullet passes the gas port move (with greater force increasing speed).......or does the BCG stay locked in place and not begin its rearward motion until the bullet passes the gas port and then move at a consistent speed?
Georgie Boy
08-16-10, 23:20
If I remember right, it's discussed in this thread somewhere: Heat is bad. DI rifles get hot. DI rifles are bad. (https://www.m4carbine.net/showthread.php?t=51218)
That was a damn good thread with eight pages filled with some really random stuff that shows what can come out of forums like this. I believe someone there said that the bolt gets pushed back against the lugs in the barrel extension until gas enters the carrier (after the bullet passes the gas port). At that point the carrier begins to move backwards, rotating/unlocking the bolt until it moves back as well.
pilotguyo540
08-16-10, 23:33
The bolt is locked closed by the diagonal slot in the bolt carrier**. The bolt cannot rotate itself free so it stays locked until the bullet passes the gas port. At this time the gas flows through the tube and pushes back on the bolt carrier. The rearward movement of the carrier allows the bolt to rotate and unlock and cycle the rest of the way.
**and also the lock tabs on the bolt and barrel extension of course
Thanks. Everytime I think I've got it a new term like "Dwell Time" comes up and the hamster starts "trying" to turn the wheel;)
Probably way more detail than you wanted but part of this post does answer your question.
http://m4carbine.net/showthread.php?t=94
FROM THIS LINK... MY QUESTIONS ADDED IN CAPS
http://m4carbine.net/showthread.php?t=94
I originally posted this on a different forum, but it got emailed around enough that someone actually emailed it back to me! Good reading, I will go through and clean it up later -- tacos wait right now!
==============================
In order to understand this, there are a few things
about the functioning of the AR that have to be
defined, I am away from most of my notes and stuff, so
most of the figures given are from memory... but
should be pretty close. For this description, the
standard rifle gas system is with the gas port located
at 13.0" and having a 20" barrel... the standard
carbine system is with the port at 7.5" and a barrel
of 14.5"
The pressures at the gas ports are: 13.5K for the
rifle and 26K for the carbine -- or twice as much.
The dwell time (the time that the gas system is
charged with high pressure) is determined by the
amount of barrel after the gas port. These are nearly
identical between the rifle and the carbine.
?DOES THE LENGTH OF THE GAS TUBE ITSELF HAVE ANY SIGNIFICANT EFFECT?
Pressure from the port is regulated only by the size
of the gas port and the diameter of the barrel.
These two factors determine the internal bolt
pressure, the maximum pressure that is obtained in the
bolt carrier/piston combination -- for the rifle this
pressure is about 1000psi and for the carbine it is
over 1500psi, half again as much.
When the rifle is fired, primer shot sets the bullet
forward until it contacts the rifling, at this point
the powder charge ignites and sets the shell case
fully back, binds the action and start to propel the
bullet. The bullet jumps slightly again and is etched
by the rifling... it stops again very briefly as the
pressures build to a point for the bullet to overcome
the mechanical advantage of the rifling twist and the
! bullet starts to spin, at this point the chamber
pressure is at max, 50K plus (there are some that
believe there is another, third stop the bullet makes
and some testing suggest this may be true).
As the chamber pressures start to climb, the brass
case expands and becomes plastic, this is essential to
seal the case in the chamber -- the correct term for
this is Obturation, when the case is obturated and
sealed, it is stuck in the chamber, practically welded
in really.
The Lock Time, or the time that the action remains
locked with no attempt to start unlocking is very
important... on the rifle, the lock time is about 550
microseconds, the lock time for the carbine is about
375 microseconds -- this may not seem like much, but
it is much shorter of a time, also keep mind that the
chamber pressures are twice as high in the carbine
when the unlocking starts.
?DOES DWELL TIME END WHEN THE BOLT UNLOCKS? OR IS THE BCG STILL CONSIDERED UNDER PRESSURE TILL IT HITS THE BUFFER?
What does all of this mean? When the carbine is fired,
the system attempts to unlock earlier than intended
and while the case is still fully obtucated... this
results in the action bind delaying the unlocking and
stressing the system. As the 5.56N is not drastically
tapered, "squirting" is not a big problem in most
guns. When the internal bolt pressures finally unlock
the bolt, the velocity of the reward movement in the
carbine is much higher than what the rifle was
designed for, it also must start extraction of the
obtucated case... as you know, the AR does not have
any sort of initial extraction, perhaps the single
biggest shortcoming of the design. This has been known
to cause ripped case heads...
At this point, as the bolt start to unlock, it is
rotated to unlock... due to much higher velocities
with the carbine, the rotation of the bolt creates
some centrifical force and helps to "float" the extractor...
the extractor on the AR is not balanced and the
forward part of it weighs more -- some argue that the
pressure of the extracted cartridge case keeps the
case head against the bolt face and test have shown
that the extractor does not generate enough force to
actually life from the case, but the fact is that the
extractor does float enough to negate some of the
spring pressure, and the contact with the case
rim becomes "soft". For this reason, it is much more
likely that the extractor will simply pop off, rather
than actually rip the case.
?IF THE CARBINE UNLOCKS FASTER THEN WOULDN'T IT INHERENTLY HAVE A SHORTER DWELL TIME?
Balanced extractors and different designs have been
developed (LMT), but the best solution to date has
been stronger extractor springs and spring buffers.
That about covers the FTE issues...
Back to bolt velocity. The high speed of the bolt has
a couple of other detrimental effects, one of the most
common is that the bolt is cycled so fast that as it
returns to battery, it actually has enough force to
"bounce" off of the barrel extension when closing and
locking... this bounce back is very small, but can be
enough to cause the weapon not to fire... this "bolt
bounce" is pretty well known.
One other problem is that the bolt can cycle so fast
the magazine spring can not keep up with it and the
round stack is not properly aligned and forced back
into place before the bolt returns to batter --
therefore there is no new cartridge picked up and the
bolt closes on an empty chamber, this is what some
call "ghost loading", or bolt-over-base jams... this
is far worse in full auto fire as the bolt does
actually move faster in full auto than semi auto; this
is due to the fact that the top cartridge in the
magazine does not apply force to the bottom to the
bolt causing drag.
The common solution to this issue is to use a stronger
recoil spring and a heavier buffer... this works, but
is treating the symptom, not the problem.
PigTail and expansion chamber gas tubes attempt to
fool the rifle into thinking that the gas port is,
located further away that it really is, but they are
not as good of a solution as actually moving the gas
port out...
I guess that about covers it for a quick rundown, of
course all of this is not nearly as simple as it
sounds.
The bolt is locked closed by the diagonal slot in the bolt carrier**. The bolt cannot rotate itself free so it stays locked until the bullet passes the gas port. At this time the gas flows through the tube and pushes back on the bolt carrier. The rearward movement of the carrier allows the bolt to rotate and unlock and cycle the rest of the way.
**and also the lock tabs on the bolt and barrel extension of courseTo be clear- The bolt cam does not lock the bolt. What actually locks the bolt is the lugs engaging lug recesses in the barrel extension. The bolt will not unlock until it's turned. If the cam didn't turn the bolt as the carrier moved rearward, the bolt would not unlock and open.
If you removed the bolt lugs, the cam would do nothing to lock the bolt and does not even act as a safety lug in case the primary lugs should shear
FROM THIS LINK... MY QUESTIONS ADDED IN CAPS
http://m4carbine.net/showthread.php?t=94
I originally posted this on a different forum, but it got emailed around enough that someone actually emailed it back to me! Good reading, I will go through and clean it up later -- tacos wait right now!
==============================
In order to understand this, there are a few things
about the functioning of the AR that have to be
defined, I am away from most of my notes and stuff, so
most of the figures given are from memory... but
should be pretty close. For this description, the
standard rifle gas system is with the gas port located
at 13.0" and having a 20" barrel... the standard
carbine system is with the port at 7.5" and a barrel
of 14.5"
The pressures at the gas ports are: 13.5K for the
rifle and 26K for the carbine -- or twice as much.
The dwell time (the time that the gas system is
charged with high pressure) is determined by the
amount of barrel after the gas port. These are nearly
identical between the rifle and the carbine.
?DOES THE LENGTH OF THE GAS TUBE ITSELF HAVE ANY SIGNIFICANT EFFECT?
Pressure from the port is regulated only by the size
of the gas port and the diameter of the barrel.
These two factors determine the internal bolt
pressure, the maximum pressure that is obtained in the
bolt carrier/piston combination -- for the rifle this
pressure is about 1000psi and for the carbine it is
over 1500psi, half again as much.
When the rifle is fired, primer shot sets the bullet
forward until it contacts the rifling, at this point
the powder charge ignites and sets the shell case
fully back, binds the action and start to propel the
bullet. The bullet jumps slightly again and is etched
by the rifling... it stops again very briefly as the
pressures build to a point for the bullet to overcome
the mechanical advantage of the rifling twist and the
! bullet starts to spin, at this point the chamber
pressure is at max, 50K plus (there are some that
believe there is another, third stop the bullet makes
and some testing suggest this may be true).
As the chamber pressures start to climb, the brass
case expands and becomes plastic, this is essential to
seal the case in the chamber -- the correct term for
this is Obturation, when the case is obturated and
sealed, it is stuck in the chamber, practically welded
in really.
The Lock Time, or the time that the action remains
locked with no attempt to start unlocking is very
important... on the rifle, the lock time is about 550
microseconds, the lock time for the carbine is about
375 microseconds -- this may not seem like much, but
it is much shorter of a time, also keep mind that the
chamber pressures are twice as high in the carbine
when the unlocking starts.
?DOES DWELL TIME END WHEN THE BOLT UNLOCKS? OR IS THE BCG STILL CONSIDERED UNDER PRESSURE TILL IT HITS THE BUFFER?
What does all of this mean? When the carbine is fired,
the system attempts to unlock earlier than intended
and while the case is still fully obtucated... this
results in the action bind delaying the unlocking and
stressing the system. As the 5.56N is not drastically
tapered, "squirting" is not a big problem in most
guns. When the internal bolt pressures finally unlock
the bolt, the velocity of the reward movement in the
carbine is much higher than what the rifle was
designed for, it also must start extraction of the
obtucated case... as you know, the AR does not have
any sort of initial extraction, perhaps the single
biggest shortcoming of the design. This has been known
to cause ripped case heads...
At this point, as the bolt start to unlock, it is
rotated to unlock... due to much higher velocities
with the carbine, the rotation of the bolt creates
some centrifical force and helps to "float" the extractor...
the extractor on the AR is not balanced and the
forward part of it weighs more -- some argue that the
pressure of the extracted cartridge case keeps the
case head against the bolt face and test have shown
that the extractor does not generate enough force to
actually life from the case, but the fact is that the
extractor does float enough to negate some of the
spring pressure, and the contact with the case
rim becomes "soft". For this reason, it is much more
likely that the extractor will simply pop off, rather
than actually rip the case.
?IF THE CARBINE UNLOCKS FASTER THEN WOULDN'T IT INHERENTLY HAVE A SHORTER DWELL TIME?
Balanced extractors and different designs have been
developed (LMT), but the best solution to date has
been stronger extractor springs and spring buffers.
That about covers the FTE issues...
Back to bolt velocity. The high speed of the bolt has
a couple of other detrimental effects, one of the most
common is that the bolt is cycled so fast that as it
returns to battery, it actually has enough force to
"bounce" off of the barrel extension when closing and
locking... this bounce back is very small, but can be
enough to cause the weapon not to fire... this "bolt
bounce" is pretty well known.
One other problem is that the bolt can cycle so fast
the magazine spring can not keep up with it and the
round stack is not properly aligned and forced back
into place before the bolt returns to batter --
therefore there is no new cartridge picked up and the
bolt closes on an empty chamber, this is what some
call "ghost loading", or bolt-over-base jams... this
is far worse in full auto fire as the bolt does
actually move faster in full auto than semi auto; this
is due to the fact that the top cartridge in the
magazine does not apply force to the bottom to the
bolt causing drag.
The common solution to this issue is to use a stronger
recoil spring and a heavier buffer... this works, but
is treating the symptom, not the problem.
PigTail and expansion chamber gas tubes attempt to
fool the rifle into thinking that the gas port is,
located further away that it really is, but they are
not as good of a solution as actually moving the gas
port out...
I guess that about covers it for a quick rundown, of
course all of this is not nearly as simple as it
sounds.
I think the second to the last paragraph partly answers one of your questions
pilotguyo540
08-18-10, 20:05
To be clear- The bolt cam does not lock the bolt. What actually locks the bolt is the lugs engaging lug recesses in the barrel extension. The bolt will not unlock until it's turned. If the cam didn't turn the bolt as the carrier moved rearward, the bolt would not unlock and open.
If you removed the bolt lugs, the cam would do nothing to lock the bolt and does not even act as a safety lug in case the primary lugs should shear
Right. All the cam does is rotate the bolt. But without the can the lock tabs would never engage. It actually never occurred to me that this would need clarification. Now everyone knows :)
To the OP:
The length of the gas tube would have to affect the dwell time. I think it would be important to know exactly when dwell time ends, but either way we know it takes longer (about 2x) to unlock the bolt on a rifle gas system. I would GUESS about half of the difference is due to the distance from the chamber and the other half time to build enough pressure to push the bolt carrier back.
As far as when dwell ends, I don't know for sure, but the second half of that question shows a little confusion you may have. The bolt does not contact the buffer. The bolt carrier does. For all intents and purposes, it is in contact with the buffer at all times.
Does the carbine have a shorter dwell time because it unlocks faster??? Again we would need to know when dwell ends. If dwell ends when the bullet. Leaves the muzzle, the dwell time for both rifles would be close with the rifle having a minuscule amount less dwell due to the increased muzzle velocity in the 20 inch barrel. If dwell time ends when the bolt opens. The rifle has twice the dwell time.
I don't think it really matters. The important thing to keep in mind is how much more violent the carbine action is in comparison to the rifle length gas system.
GO MIDDY!!! YAY!!!
Right. All the cam does is rotate the bolt. But without the can the lock tabs would never engage. It actually never occurred to me that this would need clarification. Now everyone knows :)
To the OP:
The length of the gas tube would have to affect the dwell time. I think it would be important to know exactly when dwell time ends, but either way we know it takes longer (about 2x) to unlock the bolt on a rifle gas system. I would GUESS about half of the difference is due to the distance from the chamber and the other half time to build enough pressure to push the bolt carrier back.
As far as when dwell ends, I don't know for sure, but the second half of that question shows a little confusion you may have. The bolt does not contact the buffer. The bolt carrier does. For all intents and purposes, it is in contact with the buffer at all times.
Does the carbine have a shorter dwell time because it unlocks faster??? Again we would need to know when dwell ends. If dwell ends when the bullet. Leaves the muzzle, the dwell time for both rifles would be close with the rifle having a minuscule amount less dwell due to the increased muzzle velocity in the 20 inch barrel. If dwell time ends when the bolt opens. The rifle has twice the dwell time.
I don't think it really matters. The important thing to keep in mind is how much more violent the carbine action is in comparison to the rifle length gas system.
GO MIDDY!!! YAY!!!"See, Capt'n, that's why, although you fly it, it's my airplane. You ride her hard, use her, abuse her and at the end of the day, it's this lowly airman that sweats and bleeds over her until she's put back to rights. I understand her. And trying to translate what you told me is wrong into what's really wrong adds time to the troubleshooting. You'll tell me the hydraulics don't lock the landing gear (when I know the hydraulics weren't designed to lock the landing gear that they lock mechanically) when the real trouble is the the landing gear light didn't go out because the position switch is bad"
They are locking lugs not tabs. Not everyone knows that all the cam does is rotate the bolt or somebody wouldn't have made the claim that the cam along with the locking lugs locks the bolt.
The dwell time is measured from the time the bullet clears the gas port until it exits the muzzle. Once the bullet clears the muzzle the gas pressure drops. Self loading small arms are designed that the bolt does not extract the spent casing until the pressures in the barrel have dropped to safe levels.
Carbine length gas systems have their ports closer to the chamber where pressures are much higher than they are further down the bore. In a 14.5" barrel, the distance from port to muzzle is shorter than it is with a 16" barrel. The BCG is exposed to these raised pressures for a longer time. Again, that's because the bolt must stay locked until pressures in the barrel drop to safe levels. If you used a carbine length gas system with an 18" or 20", dwell times are even longer.
Sure, Go Middy, YAY! But all you have to know capt'n is how much more violent the middy is compared to the rifle length system because you can see it doesn't really matter. Except to those who have made an effort to understand how it all works so they can be kept running
pilotguyo540
08-22-10, 20:09
"See, Capt'n, that's why, although you fly it, it's my airplane. You ride her hard, use her, abuse her and at the end of the day, it's this lowly airman that sweats and bleeds over her until she's put back to rights. I understand her. And trying to translate what you told me is wrong into what's really wrong adds time to the troubleshooting. You'll tell me the hydraulics don't lock the landing gear (when I know the hydraulics weren't designed to lock the landing gear that they lock mechanically) when the real trouble is the the landing gear light didn't go out because the position switch is bad"
They are locking lugs not tabs. Not everyone knows that all the cam does is rotate the bolt or somebody wouldn't have made the claim that the cam along with the locking lugs locks the bolt.
The dwell time is measured from the time the bullet clears the gas port until it exits the muzzle. Once the bullet clears the muzzle the gas pressure drops. Self loading small arms are designed that the bolt does not extract the spent casing until the pressures in the barrel have dropped to safe levels.
Carbine length gas systems have their ports closer to the chamber where pressures are much higher than they are further down the bore. In a 14.5" barrel, the distance from port to muzzle is shorter than it is with a 16" barrel. The BCG is exposed to these raised pressures for a longer time. Again, that's because the bolt must stay locked until pressures in the barrel drop to safe levels. If you used a carbine length gas system with an 18" or 20", dwell times are even longer.
Sure, Go Middy, YAY! But all you have to know capt'n is how much more violent the middy is compared to the rifle length system because you can see it doesn't really matter. Except to those who have made an effort to understand how it all works so they can be kept running
Wow crew chief. I am not quite sure how to interpret this. Thank you for the corrections. Always appreciated, honestly.
Flying is just another hobby for me. My profession is keeping extremely complicated machines operating. I am a field service engineer. I still bust ass in the field covered in nasty crap. I understand multiple system cooperation quite well. Responding to this post was as much to reinforce my knowledge base as to respond to the OP's question. You don't know what you don't know until you explain it, step by step in detail. What knowledge I lacked, I made no attempt to disguise. I was quite open in what I don't know.
I appreciate the effort that goes into knowing everything that is required about everything in a system to keep it running. Its what I do too. I was in no way trying to derail the knowledge train. I was just chiming in with what I know.
I fail to see how the cam does not have a part in locking the bolt when the rotation of the bolt allows the lugs to engage. Please excuse my poor parts I'd. Using incorrect terminology does not invalidate the information stated.
Thanks for the clarification on dwell definition. Now that we know dwell times biggest factor is the amount of barrel after the gas port I have a question. Will the bolt open in either system with any popular barrel length, before the bullet leaves the barrel? You stated that the bolt will not open until pressures drop to a safe level. How is this compensated for in production rifles of differing gas tube lengths? With the increased cycle rate of the carbine I would guess that maybe they are not.
Btw, the monologue was great, just pointed at the wrong guy ;)
I must tell you I appreciate you taking it well, pilotguyo540. Thanks for being a stand up guy.
Think of the deadbolt on your front door. You put the key in the slot and turn it one way and the door locks. Turn it the other way and it unlocks. The key and the mechanism slide the bolt into the recess. The key and the mechanism does not lock the door shut. It merely moves the bolt into it's recess. Remove the key and the bolt is still locked into it's recess. Kick at the door and it's the bolt in it's recess that locks everything place.
In this analogy, the cam and the carrier are the key and mechanism. The bolt is- well, the bolt.
The bolt lugs engage the recess in the barrel extension. When they do that is when the action is locked. If you were able to remove the cam and the carrier, the action would still be locked.
The bolt must remain locked until pressures drop to safe levels or things will get very exciting for the shooter. As a technician yourself, you know what happens to folks when high pressure gasses meet mortal flesh. That's what makes case ruptures and kabooms so deadly is the gasses escaping at the action end before pressures drop to a safe level.
Looking at the design of the AR, it seems that it's a triad of reciprocating mass, spring rate and cam timing that keeps the bolt locked until it's safe.
We talk about the direct impingement system of the AR as though it lacked a piston & cylinder. This isn't so. Instead of placing the piston & cylinder on the barrel, Stoner moved it inside the action. The bolt is the piston and the bolt carrier is the cylinder. Because of it's placement, it's reciprocating mass has less effect on the barrel. Clever, eh?
Cam Timing. Take a look at the slot the cam sets in. It's straight for a bit then turns. The bolt does not start unlocking until the cam rides the turned portion of the carrier. During the time the cam rides in the straight section of the slot, the bolt remains locked
Reciprocating Mass- The larger the mass, the more energy it takes to move it. (Also the greater the energy to stop it!) The same amount of gas on a bolt carrier and buffer with little mass will accelerate them more quickly than it will a bolt carrier and buffer with greater mass. It will take more time to move them and this will delay when the bolt unlocks.
Spring Rate- Higher spring rates will resist the motion of the bolt carrier group and delay the unlocking of the bolt.
The trick is to balance all three of these things. Too much mass or too high of a spring rate and the rifle will short stroke. Not enough mass or spring rate and the bolt carrier group will beat your rifle to scrap. Cam slot profile is important as well. Unlock the bolt too early at your peril.
I believe carbines use a heavier buffer and stiffer spring rates than rifles, but I am unsure.
Now add into the mix gas pressure and volume which is controlled by gas port size and location and dwell time. Fortunately, the AR gas system seems to be fairly forgiving of changes
"Capt'n, she's on the pad ready for ya. Try to bring her back in one piece this time!" :dance3:
catargadelendaest
08-29-10, 12:38
Thanks Mistwolf.
I've done a bit of thinking on Dwell Time and realized that I was getting too focused on the dwell time in which the system is charged with high pressure gas.
Dwell Time defines the time something sorta just sits in one spot. A good example is the piston in your car engine. It moves to top dead center and stops. It's dwell time is from the moment the crank shaft & connector rod pushes the piston to TDC and the piston stops to the moment the crank & connector yanks the piston back down. The same thing happens when the piston reaches bottom dead center. During those dwell times, the piston just sits there moving neither up or down.
The bolt and carrier do the same thing during the operation of the action. So yes, the bolt does have dwell time as well.
My definition of Dwell Time was confined to that which describes the time it takes the bullet to travel from gas port to muzzle. I apologize for any confusion this created
Powered by vBulletin® Version 4.2.5 Copyright © 2024 vBulletin Solutions, Inc. All rights reserved.