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Thread: Hypothetical: What Still Needs Fixing?

  1. #21
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    Quote Originally Posted by MistWolf View Post
    Not so. Increase the length of the leade and change the powder accordingly and you'll see an increase in velocity without an increase in pressure.

    Bufford, to use a longer case, you'll need to redesign the mag to accommodate the extra length to see any practical gain
    How much can you increase the leade? .01"? .60"?

    No, the leade can't be more than it is in the M16 chamber or you start to see a drop in accuracy and barrel life (shot out barrels have the origin of the rifling moved forward due to erosion among other things, so moving the origin forward is basically the same as pre-wearing out the barrel).

    How much slower can you make the powder before you start to exceed the maximum port pressure? The powders that we have now are pushing the pressures to get the velocities we want. To put it a different way - What powders will give better performance out of this cartridge than we are getting today? None, because if there were such a powder, we'd be using it.

    Unless some genius designs a powder that instantly reaches maximum pressure, holds flat at the pressure for about 250 microseconds then drops instantly to 12,000 psi and holds that for the next 300 microseconds then drops to zero, we reached the limits of this cartridge-bolt design.

    Make a better bolt, you get a little bit more performance, or better bolt life, pick one.

  2. #22
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    Quote Originally Posted by lysander View Post
    How much slower can you make the powder before you start to exceed the maximum port pressure?
    What determines the maximum port pressure? Why not use a smaller gas port and a higher port pressure to make the integral of P dV in the expansion chamber equal? This would result in equal BCG/buffer energies.

  3. #23
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    Quote Originally Posted by StainlessSlide View Post
    What determines the maximum port pressure? Why not use a smaller gas port and a higher port pressure to make the integral of P dV in the expansion chamber equal? This would result in equal BCG/buffer energies.
    Equal average energy. Peak energy would still be higher.
    "That thing looks about as enjoyable as a bowl of exploding dicks." - Magic_Salad0892

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  4. #24
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    The military specified that the leade of the 5.56 was to be longer than that of the 223 Remington in order to get more velocity while keeping pressure about the same. When the spec was set, the bullet to be used was the 55 ge FMJBT which isn't a very long bullet, or they might have specified an even longer leade. Service Rifle competitors do the same thing to allow them to load the longer 90 gr bullets with increased velocities. Roy Weatherby used the same trick to get more velocity from his line of proprietary calibers.

    I don't know what the practical limit is, but the bottom line is it's proven to work
    Last edited by MistWolf; 04-29-15 at 22:05.
    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
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  5. #25
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    I'm happy with mine they way they are.

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    Quote Originally Posted by MistWolf View Post
    The military specified that the leade of the 5.56 was to be longer than that of the 223 Remington in order to get more velocity while keeping pressure about the same. When the spec was set, the bullet to be used was the 55 ge FMJBT which isn't a very long bullet, or they might have specified an even longer leade. Service Rifle competitors do the same thing to allow them to load the longer 90 gr bullets with increased velocities. Roy Weatherby used the same trick to get more velocity from his line of proprietary calibers.

    I don't know what the practical limit is, but the bottom line is it's proven to work
    The point you are missing, or avoiding, is that every cartridge has a point where its performance peaks and can advance no further unless you are willing to increase the chamber pressure. You cannot get .338 Lapua Magnum performance out of a .308 Winchester case, unless you jack up the chamber pressure to unacceptable levels, that's why long range sniping have adopted the former.

    The 5.56mm military cartridge is at that point where if you need to have significantly higher performance, you going to need a bigger cartridge...

  7. #27
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    Quote Originally Posted by BufordTJustice View Post
    Quote Originally Posted by StainlessSlide View Post
    What determines the maximum port pressure? Why not use a smaller gas port and a higher port pressure to make the integral of P dV in the expansion chamber equal? This would result in equal BCG/buffer energies.
    Equal average energy. Peak energy would still be higher.
    There is another problem - mass flow.

    A smaller port means less mass flow through the gas system. Since the time duration of the gas pressure is limited, there are boundaries on the amount mass that can enter the system. Further, it is the expansion of the gas that performs the work so the change in volume available is directly related to the initial density (mass and pressure). So, what you actually wind up with is higher peak loads from the higher pressure, but less actual work done due to the lower mass flow through the port.

    You can move the port further toward the chamber to increase the time the port is pressurized to try and compensate for the lower mass flow, but the port pressure rises accordingly, so the port must be further restricted putting you back where you started, or the gas system need to be beefed up to handle the increased loads.

    Yes you can do it, but you would have to re-design the whole gas system and all the reciprocating parts, basically, start with a clean sheet...

  8. #28
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    I disagree. The integral is P dV, and I can get the same average P (averaged over V, not t) with a higher port pressure and a smaller port.

    You say that "volume available is directly related to the initial density (mass and pressure)" , but dV in this case is the change in volume of the expansion chamber (the gas tube is not changing volume), and doesn't depend on the gas state at all. The gas state goes into the P factor through the equation of state.

    If you want to talk about it in terms of mass flow ( which again, influences P through the equation of state), I can get any desired mass flow through the port by changing the port pressure. I can't think of a case where mass flow through an orifice wouldn't be monotonic increasing with respect to the pressure difference across the orifice.

    Maybe it would help to define my variables

    P - pressure of gas in the gas system (not the barrel or atmosphere), a function of time

    V - volume of the gas system = volume of the expansion chamber plus volume of the gas tube, also a function of time
    Last edited by StainlessSlide; 04-30-15 at 09:21. Reason: definition

  9. #29
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    Aside from the gas port arguement...

    If the question is just asking about the AR platform in general, I like to believe that there are enough aftermarket accessories to change anything about it the end user would like. Bolt catch redesign? BAD lever. Charging handle location? Side charger uppers (I'm honestly not sure if they make non reciprocating side chargers, but I don't doubt it.)

    If we are talking about the M4 TDP, that's a whole other discussion.

  10. #30
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    Quote Originally Posted by lysander View Post
    The point you are missing, or avoiding, is that every cartridge has a point where its performance peaks and can advance no further unless you are willing to increase the chamber pressure
    I do acknowledge that fact. That's why I said "I don't know what the practical limits are"


    You cannot get .338 Lapua Magnum performance out of a .308 Winchester case, unless you jack up the chamber pressure to unacceptable levels, that's why long range sniping have adopted the former.
    What about a WSM case?
    The 5.56mm military cartridge is at that point where if you need to have significantly higher performance, you going to need a bigger cartridge...
    Do you have empirical data showing that increasing the leade of the chamber will not allow enough of a velocity increase for a 75 gr bullet loaded to an OAL that allows feeding from a magazine to achieve at least 3100 fps?
    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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