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Thread: air volume and pellet myth

  1. #1
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    Default air volume and pellet myth

    for a non-regulated PCP...

    with and without pellets... are they getting the same number of shots from 180bar shoot down to 120bar... ?

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  2. #2
    Sharp Shooter

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    WHAT?
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  3. #3
    Sharp Shooter
    Doctor BAM

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    boondock
    I do not have all the answers to your questions, but would treat the question as follows:
    1) You get varying speed and therefore pressure generated by each pellet weight in a specific rifle...
    2) It is more noticeable in a spring air rifle than a PCP.....
    3) Each type of pellet will have its own POI and you must select the pellet that best performs in your rifle...
    4) When I select a pellet for a spring rifle, I calculate the highest Energy (vel x vel x mass / 450240) obtainable and take 90% of that value for consistent shooting...
    5) Above value for a PCP could be closer to 98% just because of its efficiency in using the air...
    6) In a PCP it is essential to obtain a "sweet-spot fill pressure" because the rifle will perform better and under less stress, more silent and also more economical with air...
    7) It is vital to use good quality pellets for FT, HFT and pure hunting, just to be able to repeat each shot placement for a kill.... To use cheap pellets and then blame the rifle if there is bad grouping, just does not make sense.....!

    Malan :dunno
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  4. #4
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    Not sure I understand the question, unregulated PCP sweet spot depends mostly on spring balance between hammer and valve spring in my (admittedly limited) experience. Putting in a stronger hammer spring will let out more air with each shot and move the sweet spot towards the high end of the fill pressure range, whereas a weaker hammer spring will move the sweet spot towards the lower end of the fill pressure range.

    Over the sweet spot range of consistent shots, approximately the same volume of air is released with each shot, it makes sense. At higher pressure it is harder for the hammer to hit the valve open, so the valve is opened a small amount but the air is at higher pressure. At lower pressures the valve is hit open a lot further and is kept open for longer, but the air is at a lower pressure.

    Volume of air released in a specific time period is what would determine the energy transferred to the pellet.

    I'm no scientist, just what I've been figuring out while the gears turn in my head, so I'm open to correction.
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  5. #5
    ice's Avatar ice
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    Heavy pellets = most shots !
    No pellets = least amount of shots / charge !
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  6. #6
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    This also makes sense, a heavier pellet will react more slowly to the force acting on it (the expanding air), therefore the volume made available for the air to expand into is smaller as the pellet is not as far along the barrel as a lighter pellet would be while the valve is opening and closing, therefore less air is released with each shot.
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  7. #7
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    thx for feedback... so the resistance caused by tight fitting pellets or bigger mass(heavy) pellets actualy helped to get decent shot counts?

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  8. #8
    MOT: Thomson Pneumatic Rifles

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    Martin, has it mostly right.

    Pellet velocity would remain constant (within reason) if the air volume remains the same ie 330cc free air volume at 200 bar compression (valve is releasing 1.65cc at 200 bar) or 330cc free air at 150 bar (valve is releasing 2.2cc at 150 bar), your exit pressure at the muzzle will be the same, the only major differnce will be barrel time, the higher pressure will "expand" faster so giving you a shorter barrel time.
    No pellet in the barrel will give you more shots. The reason, without a pellet to create back pressure (a pellet will only begin to move froward after a critical pressure is reached, this varies from pellet to pellet, as how tightly the pellet seats in the bore the mor inertia it has) by inertia, which holds the valve open for a fraction longer. So without a pellet the valve just spits out a small air volume.
    Heavy pellets have more inertia so hold the valve open longer, a heavy pellet is also more efficient in its use of available pressure/ volume as it travels slower alowing the available energy from the air to transfer kinetic energy into the pellet.

    More on the barrel time, that is why I tune BAMs for FT to run at max fill pressure (195bar to 145 bar +-60 shots), the shorter barrel time (known as lock time in firearms) provides less time for shooter error.

    This is the type of threads I like on the forum!!!
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  9. #9
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    [quote1149658437=ice]
    Heavy pellets = most shots !
    No pellets = least amount of shots / charge !
    [/quote1149658437]

    I don't think so :thumbsdown
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  10. #10
    Sharp Shooter
    Doctor BAM

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    Have a look at this demo, especially when you are considering spring air rifles.... PCP's are milder, but the same principles will apply...

    www.arld1.com/rifledynamicssmaller.html

    Malan at
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  11. #11
    ice's Avatar ice
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    Back pressure from pellet may be a contributing factor, but not the only thing
    in determining valve time. Hammer weight, hammer spring,
    valve return spring and transfer port size is very much part
    of this equation.

    As the hammer hits valve, high pressure air is being released in increasing
    volume as the valve is being opened further on the forward cycle of hammer and
    it keep on doing so until end of valve/hammer cycle is reached.

    Critical pressure (moving pellet) is reached even before the valve cycle completed and
    remaining higher pressure in cylinder + valve return spring closes the valve as
    the hammer return. High pressure air between valve and pellet accelerate it down the barrel.

    Without pellet the air is free to take the easy/open way out during hammer/valve cycle and more air is released as the flow from high to lower pressure is unrestricted.

    Pellet cause build-up of air during the hammer/valve cycle .Air will flow slower
    as pressure behind pellet increase until this pressure overcome resistance of pellet in barrel,
    The air from cylinder is not free to exit and air being released from cylinder have to overcome pellet resistance before it can exit (less air being released because of restriction).

    Pellet in barrel may increase valve time, but is countered by slower flow of air due to increased pressure behind pellet.
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  12. #12
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    Ice, your description is the way my little brain figures it out too, makes sense to me.

    With heavier pellet valve may stay open longer but flow of air is impeded by pellet, the air has nowhere to go.

    Pressure gradient across valve is much greater with no/lighter pellet, therefore more airflow, therefore fewer shots per fill.
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  13. #13
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    Now won't someone with a chrony please put us all out of our misery and test this theory with heavy and light pellets and let us know how right/wrong we are.

    I'm PCPless at the moment, so even my rough POI tests on paper target cannot be performed to prove/disprove the theory.
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  14. #14
    MOT: Thomson Pneumatic Rifles

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    Ice/ Huffnpuff, put this info through your super computers.
    Firstly when you fill your PCP rifle from a dive tank at 232 or 300 bar. If there is residual air pressure in your rifle, say 100bar, if you open the valve slowly the pressure builds untill 100bar and then opens your rifles valve. OK, now open the dive tank valve quite quickly and you will see that the same 100 bar residual fill will cause the gauge to spike at say +-120 bar before the rifle valve opens. The air first has to fill the "empty" pipe/ coupler volume and only then dose pressure increase!

    So as the hammer hits the valve the quicker the pellet releases the less air flow before the valve shuts, its not simply a case of the hammer hitting the valve stem, its the complete dwell time of the valve being open letting more air VOLUME through. The pressure does not equalise instantly, if this was the case our unregulated PCP's would have horrible shot counts! Say you have (using the BAMS porting area) +-0.40cc between the valve seat and the base of the pellet (I hope I am explaining this so its understandably !!!?) The valve opens and fills the void until critical release pressure of the pellet and the back pressure created by the pellets inertia in the barrel ceases, the pressure tube pressure overcomes the hammer and closes the valve (with assistance of the valve spring, another way of keeping the valve open longer), as the pressure in the rifles PT drops so the back pressure created by the pellet and the hammer impact (also inertia) keeps the valve open longer allowing a very similar AIR Volume of air through and AIR VOLUME is what its about. Even when the PT pressure has dropped to 160bar from 190 bar the velocities with heavier pellets remain quite constant (see below), although common sense tells us that a 30 bar drop should show more velocity loss, especially if the if the projectile is heavier. Also compare the energy figures below at the various shot counts, while the light pellet shows an almost straight line drop in fps and fpe the heavy pellets almost reaches and equilibrium. Therefore if the heavy pellet shows more efficiency in fps and fpe from the same charge of air it must be more efficient on air consumption i.e. shot count!

    HuffnPuff, you asked some one with a crono to test this, I have with a test rifle I built on a AR78 action and have done many velocity tests with a gauge attached to check air consumption etc. I have also do quite a few test with the BAM, below is one example that seams to bear out the above
    Shot 7.9 gn Energy
    Number Pellet
    1 1070 20.09
    2 1073 20.20
    3 1067 19.98
    4 1065 19.90
    5 1066 19.94
    6 1058 19.64
    7 1058 19.64
    8 1056 19.57
    9 1046 19.20
    10 1049 19.31
    11 1055 19.53
    12 1045 19.16
    13 1045 19.16
    14 1043 19.09
    15 1040 18.98
    16 1035 18.80
    17 1030 18.61
    18 1028 18.54
    19 1027 18.51
    20 1024 18.40
    21 1020 18.26
    22 1013 18.01
    23 1011 17.93
    24 1007 17.79
    25 1007 17.79
    26 998.8 17.50
    27 993 17.30
    28 990.5 17.21
    29 987.7 17.12
    30 981 16.89
    31 980.3 16.86
    32 970.8 16.54
    33 964.2 16.31
    34 960.7 16.19
    35 957.2 16.08
    36 949.8 15.83
    37 943.5 15.62
    38 933.5 15.29
    39 931 15.21
    40 921.7 14.91

    Shot 9.5gn Energy
    Number Pellet
    1 961.6 19.51
    2 979.9 20.26
    3 982.9 20.38
    4 982.5 20.37
    5 971.6 19.92
    6 994.2 20.86
    7 995.4 20.91
    8 1009 21.48
    9 1001 21.14
    10 999.8 21.09
    11 993.3 20.82
    12 1002 21.18
    13 1011 21.57
    14 1010 21.52
    15 1018 21.87
    16 1017 21.82
    17 1023 22.08
    18 1029 22.34
    19 1037 22.69
    20 1034 22.56
    21 1027 22.25
    22 1016 21.78
    23 1021 22.00
    24 1025 22.17
    25 1019 21.91
    26 1023 22.08
    27 1029 22.34
    28 1021 22.00
    29 1023 22.08
    30 1019 21.91
    31 1022 22.04
    32 1014 21.69
    33 1015 21.74
    34 1008 21.44
    35 1012 21.61
    36 1002 21.18
    37 999.1 21.06
    38 999.5 21.08
    39 995.8 20.92
    40 988.5 20.62

    Sorry I wanted to get the figures side by side but the forum format wil not let me, but if you compare shot numbers you will see what I mean.

    Your comments gents,
    Cheers
    Warrick
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  15. #15
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    I wish I knew what the hell you talking about!!!
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