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Thread: The Essentials of a Precision AR-15

  1. #41
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    Quote Originally Posted by Failure2Stop View Post
    Ok, finally rested from my 24 hours in airplanes and airports, and have access to my source data.

    I can't get too specific, as there are proprietary matters, however, I can disclose that we use high-speed video and pressure sensors that are accurate to silly small measurements, and have really smart engineer guys that translate that data for knuckle-draggers like me. I was corrected in that speed of sound was the local speed of sound, not the ambient speed of sound. Mea culpa.

    The collected physical data shows that the bolt carrier begins rearward movement in about twice the time it takes for the projectile to travel from the chamber to the muzzle.

    As the carrier begins its rearward movement, it travels approximately 0.075" before the cam-pin hits the first angle of the cam path, which gives additional delay to unlocking.
    Ok here is my understanding of what happens in an AR in less that the blink of an eye. Explain what I am getting wrong

    cartridge is fired in the chamber and the bullet starts its travel down the barrel

    bullet passes the gas port in the bore and instantaneously 20,000 psi of pressure travels through the gas block, gas tube and into the bolt carrier.

    at this point pressures are so high that they actually push the bolt into the chamber as well as pushing the carrier back balancing the pressures and not allowing the bolt to unlock

    when the bullet leaves the barrel pressures drop in the chamber and bore and allow the bolt to unlock and and the carrier to travel rearward removing the spent brass.

    that is the way I understand it.

  2. #42
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    Quote Originally Posted by opsoff1 View Post

    The crown on a precison rifle barrel is HUGELY important. It is also the area that is most prone to damage from either handling, cleaning or even shooting. I have brought a fair number of barrels back from demise by very careful recrowning. All done on a lathe - always. The hand tools that are available a junk. Over time, simple gas blow by when the bullet exits the barrel imparts erosion to the crown. I use different crowns for different applications as well.

    Hope that helps.
    I would not use a hand tool to resurface a muzzle crown on a bolt action rifle, but I have had some success with the PTG muzzle crown facing tool on M1 Garand barrels. The last barrel crown I resurfaced turned a 5 MOA Garand to a sub 3 MOA shooter. The PTG tool is definitely not for the impatient or ham fisted and a lathe would be preferred, but I no longer have access to precision machines.
    Train 2 Win

  3. #43
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    The forward pressure on the bolt during expansion is less than it sounds like.
    It's pretty much just enough to reduce the contact friction that is held at the lugs from initial set-back and the case sticking in the chamber from obturation.
    If pressure is applied to the rear of the bolt, pushing it forward, that would aid in unlocking, just as what happens when the carrier is pushed rearward.

    The pressure at the gas key and gas key end of the gas tube can be tracked and timed against the timing of the pressure at the port.
    The physical data shows pressure levels rising in different parts of the barrel and gas system at measurable and consistent time intervals.
    Jack Leuba
    Director of Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  4. #44
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    Quote Originally Posted by T2C View Post
    I would not use a hand tool to resurface a muzzle crown on a bolt action rifle, but I have had some success with the PTG muzzle crown facing tool on M1 Garand barrels. The last barrel crown I resurfaced turned a 5 MOA Garand to a sub 3 MOA shooter. The PTG tool is definitely not for the impatient or ham fisted and a lathe would be preferred, but I no longer have access to precision machines.
    Have you used the Manson Muzzle Crown Refacing Tool Kit? Excellent piece of kit and in some ways better than a lathe.



    C4

  5. #45
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    In case you might need to clean your gas tube I point the barrel down at about a 45 degree angle and spray into the tube brake cleaner with the tiny tube attached to the cans push nozzle. The brake fluid should flow freely out of the barrel to be cleaned properly. I usually do this every time I am cleaning the action with brake fluid then let it dry good and lubricate like you normally would do. I have never had a problem with anything since I started doing this 15 years ago.

  6. #46
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    Quote Originally Posted by Failure2Stop View Post
    I can't get too specific, as there are proprietary matters, however, I can disclose that we use high-speed video and pressure sensors that are accurate to silly small measurements, and have really smart engineer guys that translate that data for knuckle-draggers like me. I was corrected in that speed of sound was the local speed of sound, not the ambient speed of sound. Mea culpa.
    Again - no disrespect, but I have no idea what you are trying to say. Local speed of sound? Ambient speed of sound? Ambient is the immediate surrounding - so I am assuming you are referring to the pressure and velocity of the gas at the muzzle?
    Basic chemistry and more specifically, thermodynamics as related to firearms and ammunition adhere to all the same laws of physics as does everything else in the world. With that in context, small arms propellants are classified as low order explosives. The most powerful of these low order explosives are these same smokeless powders. In low order explosives, the process of burning or "decomposition" is termed deflagration. This process produces heat, light and a subsonic pressure wave. (Much to our testosterone fueled delight) However, it is important to note that this is in an uncontained state - like pouring a pile of 4895 on the ground and sparking it up. The pressure wave is subsonic, meaning it does not produce sound waves in the infrasound (very low) or the ultra sound (very high) frequencies (neither of which is in the range of human hearing) - so all you hear is the fizzing of the burning. There is no blast. So, when uncontained - the pressure wave or sound wave presents itself as subsonic - it doesn't exceed the speed of sound (1126fps in dry air at 68 deg F) However, when contained in small vessels (cartridge cases & bores) the pressures generated can exceed the material strength of the barrel / firearm. These extreme pressures and rapid expansion of the decomposing propellant is what we see exiting from the muzzle as the bullet clears the barrel. High pressure,supersonic sound waves - give us the muzzle blast and accompanying explosive like sound report. Measurements and calculations indicate that the velocity of these gases exiting from the bore (known as the velocity of the reaction front) hover around 5200 fps for a long gun.
    The process of burning the powder and building the pressure to move the bullet down the bore is a Pressure/Volume/Temperature problem. Smokeless powder contains its own oxidizing agent - it can feed itself. More heat builds pressure, more pressure builds heat, and so on - the variable is the volume of the vessel which changes as the pressure moves the bullet down the bore.

    In your original response, you stated that the pressure wave moves at the speed of sound - this is reasonable statement for propellant that is burning in an uncontained state. However, everything changes in a closed vessel. Since we have all sorts of burn rates, single base, double base and even triple base powders, the velocity of the reaction front at the muzzle is accepted within the engineering and physics circles to be 4000 - 6000 fps. As stated above and earlier - the mean is accepted to be approximately 5200fps for a rifle. Once clear of the bore an in an uncontained state, this gas pressure front cools rapidly and dissipates.

    Quote Originally Posted by Failure2Stop View Post
    The collected physical data shows that the bolt carrier begins rearward movement in about twice the time it takes for the projectile to travel from the chamber to the muzzle.
    I had to really think about this - and this is understandable, but it is very misleading. Establishing a point of measurement from the chamber to compare carrier movement is unfair as there is nothing to move the carrier. Nothing should be moving other than the bullet in the bore until the bullet passes the gas port. Once this happens, the expanding gas enters the gas tube and expansion chamber in the carrier. In a full sized 20" AR/M16, the port is 6.825" from the muzzle. This allows for a pressurization of the bore, chamber, gas tube and carrier for only the time that the bullet is between the port and the muzzle. That is a very very short amount of time. I have seen data that indicatates it is south of 1 millisecond. As soon as the bullet clears the muzzle - the pressure drops. What allows for the whole mechanism to work is the momentum of the gas "charge". This is where the port tuning becomes so important. Port location dictates the pressure available at the port. (13,000 psi for an M16, 26,000 psi for an M4) Port diameter dictates the volume of the gas that gets sent down the tube. In a crude analogy for port diameters, if you have a baseball traveling at 1000 fps and a bowling ball traveling at 1000 fps - which is going to do more work? Conversely, when we attach a can to the muzzle - we are in effect lengthening the bore and this pressurizes the system longer and cause all sorts of issues, not the least of which is hard recoil. Again - I point to the M4 problems - all directly related to the gas port location change and the accompanying spike in port pressures.

    ETA - on the flip side and as an example of momentum - look at an M1 Garand - the port is an inch from the muzzle - that bullet barely even travels one caliber before it clears the barrel - so the propellant gas gets a RHCH nanosecond to impart a punch to the oprod - its all momentum baby!


    Quote Originally Posted by Failure2Stop View Post
    As the carrier begins its rearward movement, it travels approximately 0.075" before the cam-pin hits the first angle of the cam path, which gives additional delay to unlocking.
    I absolutely agree and understand that. This is engineered into the design This delay (aka dwell) is required to allow chamber pressures to subside in order for extraction to occur. Momentum is force multiplier! This dwell is also part of the inherent accruacy of this system. If a shooter can exhibit proper follow through with an AR type rifle, there is no mechanical disadvantage to the system other than the slow lock time, but that is up front, prior to ignition. Unfortunately this isn't the case with an M4 type system - the early bolt unlocking and high rotational speed of the bolt actually begin when there is still significant pressure in the chamber.

    As a qualifier, I am a bit reluctant to say that this isn't merely a hobby. I don't like to get into resume' competitions - but I offer this as a bit of a qualification to the information presented, this isn't stuff I pulled out of a comic book while fondling myself in my mothers basement - I work for one of the premier DOD R&D/S&T labs in the country and my section deals with ballistics, blast protection, terminal performance, body armor and individual protection and individual equipment as well as ballistic threat assessments. We live it, breathe it and eat it everyday. Hopefully the information will provoke rational thought and discussion of these wonderful little rifles.
    Last edited by opsoff1; 08-13-15 at 19:35.
    opsoff

    "I'd rather go down the river with seven studs than with a hundred shitheads"- Colonel Charlie Beckwith

  7. #47
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    Alrighty, had to tap one of my engineers for the following response since I am not an engineer.
    Some of the reply had to be edited to remove proprietary knowledge and design information.

    Quote Originally Posted by opsoff1 View Post
    Again - no disrespect, but I have no idea what you are trying to say. Local speed of sound? Ambient speed of sound? Ambient is the immediate surrounding - so I am assuming you are referring to the pressure and velocity of the gas at the muzzle?
    RD engineering analyst responding, I don’t want to hurt any feelings, my goal is to educate, but please realize I cannot give any proprietary knowledge.

    The gas field in a gas system consists of constantly varying pressures, velocities, and temperatures, where every location and point of time has a different speed of sound.

    Basic chemistry and more specifically, thermodynamics as related to firearms and ammunition adhere to all the same laws of physics as does everything else in the world. With that in context, small arms propellants are classified as low order explosives. The most powerful of these low order explosives are these same smokeless powders. In low order explosives, the process of burning or "decomposition" is termed deflagration. This process produces heat, light and a subsonic pressure wave. (Much to our testosterone fueled delight) However, it is important to note that this is in an uncontained state - like pouring a pile of 4895 on the ground and sparking it up. The pressure wave is subsonic, meaning it does not produce sound waves in the infrasound (very low) or the ultra sound (very high) frequencies (neither of which is in the range of human hearing) - so all you hear is the fizzing of the burning. There is no blast. So, when uncontained - the pressure wave or sound wave presents itself as subsonic - it doesn't exceed the speed of sound (1126fps in dry air at 68 deg F)
    Nothing wrong here other than sound emanating from this burn occurs at the speed of sound( not less than) and this has nothing to do with combustion in firearms.

    However, when contained in small vessels (cartridge cases & bores) the pressures generated can exceed the material strength of the barrel / firearm.
    This would mean that the gun is exploding, but since we know what these pressures are we are able to design for them, and alternately a light finger press could break a small enough part, but this really isn't pertinent to the discussion.

    These extreme pressures and rapid expansion of the decomposing propellant is what we see exiting from the muzzle as the bullet clears the barrel. High pressure,supersonic sound waves - give us the muzzle blast and accompanying explosive like sound report. Measurements and calculations indicate that the velocity of these gases exiting from the bore (known as the velocity of the reaction front) hover around 5200 fps for a long gun.
    The process of burning the powder and building the pressure to move the bullet down the bore is a Pressure/Volume/Temperature problem. Smokeless powder contains its own oxidizing agent - it can feed itself. More heat builds pressure, more pressure builds heat, and so on - the variable is the volume of the vessel which changes as the pressure moves the bullet down the bore.
    The combustion is an internal energy equaling PVT problem.

    Only at immediate muzzle exit do you have a transition to supersonic, based on rapid acceleration behind the initial shock wave as energy contained in this wave dissipates it slows to a simple expanding sound wave. The time scale on this event is about 10^-6s. The muzzle blast is a pressure release not an explosion, there is no additional energy being released.

    In your original response, you stated that the pressure wave moves at the speed of sound - this is reasonable statement for propellant that is burning in an uncontained state. However, everything changes in a closed vessel. Since we have all sorts of burn rates, single base, double base and even triple base powders, the velocity of the reaction front at the muzzle is accepted within the engineering and physics circles to be 4000 - 6000 fps. As stated above and earlier - the mean is accepted to be approximately 5200fps for a rifle. Once clear of the bore an in an uncontained state, this gas pressure front cools rapidly and dissipates.
    Burn rate has nothing to do with gas velocity and again at the muzzle there is no continued explosion, only the rapid expansions of gas into an ambient environment.

    I had to really think about this - and this is understandable, but it is very misleading. Establishing a point of measurement from the chamber to compare carrier movement is unfair as there is nothing to move the carrier. Nothing should be moving other than the bullet in the bore until the bullet passes the gas port. Once this happens, the expanding gas enters the gas tube and expansion chamber in the carrier.
    We have the capability to see the pressure and motion and link them to a common time accurate to 1/200,000 s. The combustion can be seen as well as time pressure response at gas port and beginning of the gas tube.

    The misconception is that everything needs to occur the same way a balloon fills, flow has inertia and once gas has started it will continue. This will occur at a predictable velocity based on pressure expanding into free air with a substantial wall friction.

    In a full sized 20" AR/M16, the port is 6.825" from the muzzle. This allows for a pressurization of the bore, chamber, gas tube and carrier for only the time that the bullet is between the port and the muzzle.
    This is another gun lore misconception, as said below gas has momentum and does not need to have a sealed system to work. Using this example, projectile in bore time is 2.29*10^-4. Gas travel time in gas tube is substantially greater.

    That is a very very short amount of time. I have seen data that indicatates it is south of 1 millisecond. As soon as the bullet clears the muzzle - the pressure drops. What allows for the whole mechanism to work is the momentum of the gas "charge". This is where the port tuning becomes so important. Port location dictates the pressure available at the port. (13,000 psi for an M16, 26,000 psi for an M4) Port diameter dictates the volume of the gas that gets sent down the tube. In a crude analogy for port diameters, if you have a baseball traveling at 1000 fps and a bowling ball traveling at 1000 fps - which is going to do more work? Conversely, when we attach a can to the muzzle - we are in effect lengthening the bore and this pressurizes the system longer and cause all sorts of issues, not the least of which is hard recoil. Again - I point to the M4 problems - all directly related to the gas port location change and the accompanying spike in port pressures.
    This is actually not the case, the analogy is much closer to stepping on a garden hose and flow decreasing.

    Ok, me back:
    The net of the discussion is this: original claim that "ARs want to jump, bounce and twist when you let a shot go, as the carrier starts to begin its cycle before the bullet exits the bore". Physical evidence proves that this is not the case unless intentionally producing a system that does so.
    Jack Leuba
    Director of Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  8. #48
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    Jack, again with respect,
    Quote Originally Posted by Failure2Stop View Post
    The gas field in a gas system consists of constantly varying pressures, velocities, and temperatures, where every location and point of time has a different speed of sound.
    Agreed - but specific to a gas system as there are restrictions (ports) vents, leaks etc. In a closed bore - like a bolt action rifle, the pressure is not linear, but rather when plotted as time vs pressure, it generates a parabolic curve.
    I am not sure what you are talking about when you say "a different speed of sound". Sound travels at a constant; 343 m/s or 1126 fps. Frequency??
    Nothing wrong here other than sound emanating from this burn occurs at the speed of sound (not less than) and this has nothing to do with combustion in firearms
    I didn’t say it was less – what I was trying to impart was specific to the frequencies of the pressure wave. We were discussing the velocity of the reaction front – AKA the pressure wave that emanates from the muzzle generates the noise we hear. The noise we hear is at “sonic” or the speed of sound levels – not supersonic or subsonic - which we can’t hear. Uncontained powder burns – it doesn’t go bang was my point.
    This would mean that the gun is exploding, but since we know what these pressures are we are able to design for them, and alternately a light finger press could break a small enough part, but this really isn't pertinent to the discussion.
    Not what I was trying to say - the context of this was that different powders can generate dangerous / catastrophic pressure levels. Do you think an AR would fare ok with a case full of N310? In more applicable terms, if we load too fast of a rifle powder, the pressure at the port is insufficient to cycle to system – and conversely, too slow a powder can impart port pressures that are far more than needed, usually resulting in damage to the rifle. (Just like M1 Garands w/ heavy bullets and slow powder – they bend op rods)
    The combustion is an internal energy equaling PVT problem.
    I stated this two posts ago “The process of burning the powder and building the pressure to move the bullet down the bore is a Pressure/Volume/Temperature problem. Smokeless powder contains its own oxidizing agent - it can feed itself. More heat builds pressure, more pressure builds heat, and so on - the variable is the volume of the vessel which changes as the pressure moves the bullet down the bore.”
    Only at immediate muzzle exit do you have a transition to supersonic, based on rapid acceleration behind the initial shock wave as energy contained in this wave dissipates it slows to a simple expanding sound wave. The time scale on this event is about 10^-6s. The muzzle blast is a pressure release not an explosion, there is no additional energy being released.
    Agreed again – however with the caveat that you used the right powder and it was consumed in the bore. Too slow a powder and you get obnoxious muzzle blasts – think of a .22LR vs .338 LM. And to be specific, I wrote: “give us the muzzle blast and accompanying explosive like sound report.”

    Burn rate has nothing to do with gas velocity and again at the muzzle there is no continued explosion, only the rapid expansions of gas into an ambient environment.
    Actually this is false – burn rates do affect the velocity of the reaction front, AKA gas velocity, as does the amount of powder being burned. I suspect that we may be trying to explain two different things here.
    We have the capability to see the pressure and motion and link them to a common time accurate to 1/200,000 s. The combustion can be seen as well as time pressure response at gas port and beginning of the gas tube.
    The misconception is that everything needs to occur the same way a balloon fills, flow has inertia and once gas has started it will continue. This will occur at a predictable velocity based on pressure expanding into free air with a substantial wall friction.
    Ok – cool.

    This is another gun lore misconception, as said below gas has momentum and does not need to have a sealed system to work. Using this example, projectile in bore time is 2.29*10^-4. Gas travel time in gas tube is substantially greater.
    So this was in response to: “In a full sized 20" AR/M16, the port is 6.825" from the muzzle. This allows for a pressurization of the bore, chamber, gas tube and carrier for only the time that the bullet is between the port and the muzzle.”
    This is actually fact – it’s not lore. I didn’t say it needed to have a sealed system to work – what I wrote was this is the only time the system gets pressurized. It doesn’t need to be sealed to operate. I wrote; “What allows for the whole mechanism to work is the momentum of the gas "charge". Maybe inertia is a more accurate term vs momentum.
    This is actually not the case, the analogy is much closer to stepping on a garden hose and flow decreasing.
    Ok – so bad analogy on my part. Bottom line – available port pressure is a function of location and gas volume (which is tied to the inertia) is a variable driven by port diameter. Too small a port at the same location and the rifle won’t cycle. Too big a port at the same location and the same rifle is overgassed and beats itself silly. The pressure doesn't change the volume of the gas through the restriction does.
    Ok, me back:
    The net of the discussion is this: original claim that "ARs want to jump, bounce and twist when you let a shot go, as the carrier starts to begin its cycle before the bullet exits the bore". Physical evidence proves that this is not the case unless intentionally producing a system that does so.
    Who have I been sparring with all this time? LOL
    Great discussion – time to ring the bell. I need to hydrate and towel off. Read Cap’t & coke and a poolside nap. Have a great weekend.
    opsoff

    "I'd rather go down the river with seven studs than with a hundred shitheads"- Colonel Charlie Beckwith

  9. #49
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    Who have I been sparring with all this time? LOL
    As I stated at the start of the post, since I am not an engineer (I'm pretty much just an educated end-user), I had one of my R&D engineers write out a response. He's one of the "smart guys" that feeds me usable data.

    Anyway, as it's closing time on Friday here I won't be able to get a detailed reply from him until Monday at soonest, and that's assuming that he isn't busy doing his real job.

    I can address this:

    Quote Originally Posted by opsoff1 View Post
    Jack, again with respect,

    Agreed - but specific to a gas system as there are restrictions (ports) vents, leaks etc. In a closed bore - like a bolt action rifle, the pressure is not linear, but rather when plotted as time vs pressure, it generates a parabolic curve.
    I am not sure what you are talking about when you say "a different speed of sound". Sound travels at a constant; 343 m/s or 1126 fps. Frequency??
    The speed of sound is variable, and constant only at specific conditions.
    True, at 68 degrees F in dry air at sea-level, the speed of sound is 343.2 meters per second, but change those parameters and the speed of sound changes.

    http://www.engineeringtoolbox.com/el...ir-d_1534.html

    This is pertinent to me, as an end-user, and I am pretty well versed in it.
    Big reason, for me, is knowing where a projectile will go trans-sonic as it distinctly affects stability characteristics.
    It's indicated in red on Applied Ballistics trajectory software with regard to the environmental inputs.
    Jack Leuba
    Director of Sales
    Knight's Armament Company
    jleuba@knightarmco.com

  10. #50
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    Quote Originally Posted by C4IGrant View Post
    Have you used the Manson Muzzle Crown Refacing Tool Kit? Excellent piece of kit and in some ways better than a lathe.



    C4
    I have used Manson reamers, but not their muzzle crown resurfacing kit. I have my eye on a LeBlond engine lathe that will be coming up for sale.
    Train 2 Win

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