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Thread: Pistons and violent early extraction...

  1. #31
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    The bolt does not begin unlocking as soon as the carrier begins moving. The groove in the carrier which engages the bolt cam is straight for a short distance, then angles which delays the unlocking of the bolt
    Yeah, I know, poor wording on my part perhaps. I didn't mean that unlocking starts when the carrier moves to the rear, I was trying to say that it was a result of the cam pin following the cam track, and the resulting rotation of both cam pin and bolt. If that makes sense.....I suck at english firearms nomenclature....sorry....

    @dano5326:

    We still have cam pin recess wear marks, if that is what you are talking about in regards to inlet:



    Don't blame your armorer.. they are constrained by the limit of education & most are lego assembling monkeys.
    Well, this guy is actually quite experienced, having been in the business for over 30 years, with several armorer courses from various manufacturers (FN, HK, Colt Canada/Diemaco, Glock). He does know his stuff.

    Look at where the piston is in the standard AR. In the middle of the bolt carrier. As gas goes through the key, into the chamber (within the carrier) it pushes in all directions.
    - The bolt will be pushed forward and the carrier rearward... gas venting out the side holes in the carrier as the gas rings pass.
    - as the carrier is pushed rearward the cam path unlocks the bolt, aligns it with openings in barrel extension
    - momentum is keeping the carrier moving rearward.
    Yeah, I understand the physical differences between the two systems. I just don't see where the bolt can go. I can see the gas inside the carrier making sure that the bolt stays forward during unlocking, but the same thing can be said about the firing pin spring on th HK bolt.
    Last edited by Arctic1; 03-13-12 at 17:53.

  2. #32
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    Quote Originally Posted by Arctic1 View Post
    Yeah, I know, poor wording on my part perhaps. I didn't mean that unlocking starts when the carrier moves to the rear, I was trying to say that it was a result of the cam pin following the cam track, and the resulting rotation of both cam pin and bolt. If that makes sense.....I suck at english firearms nomenclature....sorry....
    Gotcha. No apologies needed


    Yeah, I understand the physical differences between the two systems. I just don't see where the bolt can go. I can see the gas inside the carrier making sure that the bolt stays forward during unlocking, but the same thing can be said about the firing pin spring on th HK bolt.
    The bolt is held in place mechanically by the locking lugs during unlocking. It's also held in position within the carrier mechanically by the bolt cam. If anything holds the bolt in the forward most position in the carrier during extraction, it's inertia
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

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  3. #33
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    Quote Originally Posted by MistWolf View Post
    While the gases do push against the bolt, they cannot push the bolt forward! Although the bullet has exited the barrel by the time the expansion chamber fills with enough pressure to start the carrier moving, the pressure in the bore is still greater. Also, the spent case has expanded enough to take up what little slack there was and resist any forward motion of the bolt.

    It's also a fallacy to believe the AR does not have a piston. It's piston is part of the bolt and the carrier is a cylinder. The piston is inline with the bolt and bore, reducing muzzle climb induced by off center recoil forces and reciprocating mass.

    The AR does not feature primary extraction, a mechanical camming action that starts extraction as the bolt begins unlocking. Many other designs do, such as the M14 and most bolt actions. Whether the AR uses the original inline piston or an aftermarket offset piston relocated to the gas block, it must rely on "brute force" to yank the spent cartridge from the chamber
    ^ That in a nutshell.

  4. #34
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    Another difference that I have not seen mentioned so far is that of reduced dwell time with a piston upper.

    With a conventional DI system the gas enters the gas tube as the bullet passes the gas port, then has to travel all the way back the gas tube, enter the carrier and pressurize it before any carrier movement and extraction takes place.

    This dwell allows the obturated case to shrink away from the chamber walls.

    With a piston upper, as soon as the gas enters the gas port it can pressurize the small chamber and start the rearward piston movement immediately, which starts carrier movement and extraction pretty much right then. Obturation of the case may still be going on, which means the case is still gripping the chamber walls. Hence the extractor ripping through the rim, or just breaking.

    Of course this is exacerbated when you have a 16" barrel and shorty gas system.

  5. #35
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    Quote Originally Posted by MistWolf View Post
    There is no way the expansion chamber will pressurize enough to start moving the carrier before the bullet can travel from gas port to muzzle. The distance from gas port to muzzle is shorter than it is from gas port to carrier. Also, if I remember correctly, the velocity of the supersonic gases drop to subsonic after passing through the gas port. In the race between bullet to muzzle and gas to expansion chamber, the bullet wins every time.

    When you see flames coming out of the chamber, it's for the same reason you see flames coming out of the muzzle. It's the spent gases igniting as they mix with the oxygen in the atmosphere


    I'm guessing you are assuming the 15,000 psi gas behind the bullet moves at the same speed as the bullet which when passing by the port in the 7" location is apx 2400fps? You may want to do a little more research on that.
    Ok are the flaming gases coming out of the muzzle high pressure or do you think they flow out all mellow like? If they shoot out of the ejection port that would say there's a little pressure behind them.
    If you look at a pressure curve from a P. or PT system the 58,000 psi pressure is only while the bullet is in the first few inches by the time it passes the gas port it's 12-15000 psi and then around 8000 psi when the bullet exits depending on the caliber, powder and barrel length. after the bullet exits the pressure in the barrel drops to 0 very quick, there shouldn't be any pressure in the case so the walls of the case contract and allow it to be pulled from the chamber.
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  6. #36
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    With a piston upper, as soon as the gas enters the gas port it can pressurize the small chamber and start the rearward piston movement immediately, which starts carrier movement and extraction pretty much right then. Obturation of the case may still be going on, which means the case is still gripping the chamber walls. Hence the extractor ripping through the rim, or just breaking.
    Yeah, you would think that they didn't test this, huh?

  7. #37
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    Quote Originally Posted by tpelle View Post
    With a piston upper, as soon as the gas enters the gas port it can pressurize the small chamber and start the rearward piston movement immediately, which starts carrier movement and extraction pretty much right then.
    that's going to depend on the volume of that expansion chamber. you can't assume it's less than the volume of the gas tube and chamber inside the carrier, unless you know that it is.

  8. #38
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    Quote Originally Posted by constructor View Post
    I'm guessing you are assuming the 15,000 psi gas behind the bullet moves at the same speed as the bullet which when passing by the port in the 7" location is apx 2400fps? You may want to do a little more research on that.
    Ok are the flaming gases coming out of the muzzle high pressure or do you think they flow out all mellow like? If they shoot out of the ejection port that would say there's a little pressure behind them.
    If you look at a pressure curve from a P. or PT system the 58,000 psi pressure is only while the bullet is in the first few inches by the time it passes the gas port it's 12-15000 psi and then around 8000 psi when the bullet exits depending on the caliber, powder and barrel length. after the bullet exits the pressure in the barrel drops to 0 very quick, there shouldn't be any pressure in the case so the walls of the case contract and allow it to be pulled from the chamber.
    Exactly. On a DI AR as the gas enters the carrier key and the carrier the bolt receives forward pressure and the carrier rearward pressure as the carriers chamber fills with gas (at the same time since the bolt is the forward end of this chamber and the carrier is the rear of this chamber). When the chamber can no longer contain anymore the carrier moves rearward and this causes the bolt to unlock due to the track in the carrier (bolt connected to the carrier via cam pin) follows this track. By this time the chamber pressure has dropped so much that the empty case easily follows and stays attached to the bolt via the extractor. As the bolt reaches fully unlocked the gas rings pass by the two ports on the right side of the carrier and the gas is vented outside those ports.

    One of the reasons for the H (3.8oz) and heavier CAR length buffers are to help delay the carrier from defeating the buffer and spring as soon as it would with a CAR (3.0oz) buffer, a heavier weight buffer is harder to defeat (simple physics). The CAR length gas guns are trying the move the bolt rearward faster than a longer rifle gas system does due to the shorter length and reduced gas system capacity. Because of the shorter gas system length the internal chamber pressure on the fired casing on a CAR gas gun is higher than it is on a mid length or rifle gas length gun, thus why the heavier buffers and stronger extractors help. Some of this can be controlled via the gas port in the barrel size. But it's still a timing issue, the system has to have enough gas to fill the gas system fast enough before the bullet leaves the barrel (dwell time). The stronger extraction devices (stronger extractor springs and O-rings) help keep the empty on the bolt due to this higher pressure on the empty case.
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  9. #39
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    Quote Originally Posted by constructor View Post
    I'm guessing you are assuming the 15,000 psi gas behind the bullet moves at the same speed as the bullet which when passing by the port in the 7" location is apx 2400fps? You may want to do a little more research on that.
    Ok are the flaming gases coming out of the muzzle high pressure or do you think they flow out all mellow like? If they shoot out of the ejection port that would say there's a little pressure behind them.
    If you look at a pressure curve from a P. or PT system the 58,000 psi pressure is only while the bullet is in the first few inches by the time it passes the gas port it's 12-15000 psi and then around 8000 psi when the bullet exits depending on the caliber, powder and barrel length. after the bullet exits the pressure in the barrel drops to 0 very quick, there shouldn't be any pressure in the case so the walls of the case contract and allow it to be pulled from the chamber.
    The velocity of the ejecta is faster than that of the bullet once it uncorks. In the formula to figure the free recoil of a rifle is a constant (if I recall correctly) is 4750 fps. However, the gas port reduces the flow of the gas increasing the time needed to pressurize the expansion chamber and get the carrier moving.

    In order for the rifle to extract the spent case before the bullet clears the barrel, it would have to do so in fraction of a millisecond a that's all the time it takes the bullet to travel from gas port to muzzle
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

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  10. #40
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    Let me add that until the bullet uncorks the gases in the bore are only traveling as fast as the bullet.

    Robb, I'm pretty certain that the expansion chamber reaches operating pressures after the bullet uncorks. I believe the pressure and density of the residual gas is what the rifle is operating on
    INSIDE PLAN OF BOX
    1. ROAD-RUNNER LIFTS GLASS OF WATER- PULLING UP MATCH
    2. MATCH SCRATCHES ON MATCH-BOX
    3. MATCH LIGHTS FUSE TO TNT
    4. BOOM!
    5. HA-HA!!

    -WILE E. COYOTE, AUTHOR OF "EVERYTHING I NEEDED TO KNOW IN LIFE, I LEARNED FROM GOLDBERG & MURPHY"

    http://i115.photobucket.com/albums/n289/SgtSongDog/AR%20Carbine/DSC_0114.jpg
    I am American

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