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Thread: How to determine Foot poundage.

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    How to determine Foot poundage.

    I seem to recall way back when that to find the foot poundage of an air pistol or air rifle there was a gadget, which looked like a metal quadrant on a stand, this had if memory serves me correctly two pendulums , when placed,, a short distance down range the object was to hit a paddle on one of them, this swung the pendulum initiating the second pendulum to swing on impact.
    On the quadrant which was calibrated the second pendulum would stay at its most forward position , and the one that was hit by the pellet would return to its original position thus leaving a reading on the pendulum giving the foot poundage of the chosen weapon, am I correct and are these gadgets still around or does one now get the desired info from a chronograph? .

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    Google "Ballistic Pendulum". Was a time when this was the only way, but most people find electronic chronographs easier to use and more accurate these days.
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    Also if you have the speed and weight of pellet there are loads of apps for phone or computer that do instant calculation for you if equipment does not.

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    Speed of the pellet in feet per second, times speed of the pellet in feet per seconds, times pellet weight in grains, divide by 450240. Simples
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    Quote Originally Posted by airgunnut View Post
    Speed of the pellet in feet per second, times speed of the pellet in feet per seconds, times pellet weight in grains, divide by 450240. Simples

    Where 450240 is twice the gravity constant multiplied by grains per lb. (7000)
    You can change the constant accordingly if you weight shot in ounces or lbs.
    Another interesting number is 671. Any object travelling at 671fps has a ft/lb power the same as its weight in grains. Ie a 15.6gr AA pellet at 671 is running at 15.6 ft/lb. an 8.4gr at 8.4 ft/lb etc.
    so a 5lb cannon ball at 671fps is a whopping 35000 ft/lb, but it will still have the same basic trajectory as the .177 8.4gr pellet.
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    Quote Originally Posted by tinbum View Post
    so a 5lb cannon ball at 671fps is a whopping 35000 ft/lb, but it will still have the same basic trajectory as the .177 8.4gr pellet.
    Ah, but the .177 pellet is no good for bouncing through massed ranks of Frenchmen
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    Quote Originally Posted by tinbum View Post
    Where 450240 is twice the gravity constant multiplied by grains per lb. (7000)
    You can change the constant accordingly if you weight shot in ounces or lbs.
    Another interesting number is 671. Any object travelling at 671fps has a ft/lb power the same as its weight in grains. Ie a 15.6gr AA pellet at 671 is running at 15.6 ft/lb. an 8.4gr at 8.4 ft/lb etc.
    so a 5lb cannon ball at 671fps is a whopping 35000 ft/lb, but it will still have the same basic trajectory as the .177 8.4gr pellet.
    I think the British, as distinct from the American Standard for g results in a divisor of 450436.68 .

    The 5lb cannon ball (and who ever used a 5 -pounder? ) won't have the same trajectory as any airgun pellet because there's a helluva lot more grains per square unit of cross section in the 5lb ball, so it'll retain velocity much better. You'd probably start to notice the difference within 50 yards or so.
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    Quote Originally Posted by MikB View Post
    I think the British, as distinct from the American Standard for g results in a divisor of 450436.68 .

    The 5lb cannon ball (and who ever used a 5 -pounder? ) won't have the same trajectory as any airgun pellet because there's a helluva lot more grains per square unit of cross section in the 5lb ball, so it'll retain velocity much better. You'd probably start to notice the difference within 50 yards or so.
    Mik, for as long as I can remember, British publications have put the devisor at 450240. American sources I've seen all quote 450400. I don't know why there's a fifference.
    Do we know what figure the police/phorensic service use when/if they have to test an airgun?
    The South of England has 2 good things, the M1 and the A1. Both will take you to Yorkshire.

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    Quote Originally Posted by harry mac View Post
    Mik, for as long as I can remember, British publications have put the devisor at 450240. American sources I've seen all quote 450400. I don't know why there's a fifference.
    Do we know what figure the police/phorensic service use when/if they have to test an airgun?
    The divisor is the result of the acceleration from gravity ("g") and the number of grains in a pound, and the constant "2". The number of grains in a pound and the value "2" are fixed values. The differences arise from small differences in the value assumed for "g". "g" is not a constant all over the surface of the earth, it varies with altitude, underlying geology, and proximity to the equator, so any value applied universally will likely be an approximation to the value of "g" at your exact location. For perfect accuracy the forensic service would need to ascertain the exact value of "g" at the test location and adjust the calculation accordingly, but the differences attributable to variations in local "g" are very small (approximately .03% between the values you quote) and much smaller than the other measurement errors. To be credible, a forensic result for muzzle energy would also have to specify the maximum error in the given figure, e.g. 11.98 ft-lbf +_ 0.1%
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    Quote Originally Posted by Turnup View Post
    The divisor is the result of the acceleration from gravity ("g") and the number of grains in a pound, and the constant "2". The number of grains in a pound and the value "2" are fixed values. The differences arise from small differences in the value assumed for "g". "g" is not a constant all over the surface of the earth, it varies with altitude, underlying geology, and proximity to the equator, so any value applied universally will likely be an approximation to the value of "g" at your exact location. For perfect accuracy the forensic service would need to ascertain the exact value of "g" at the test location and adjust the calculation accordingly, but the differences attributable to variations in local "g" are very small (approximately .03% between the values you quote) and much smaller than the other measurement errors. To be credible, a forensic result for muzzle energy would also have to specify the maximum error in the given figure, e.g. 11.98 ft-lbf +_ 0.1%
    According to the latest published survey results, using the average value of "G" for the UK the constant should be approximately 450750 (actual value is about 450753 ish). If you are depending on the differences in all the values to prove your gun is legal you are probably not going to manage it. The 450240 figure usually quoted seems to be accurate in Salt Lake city.

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    Been thinking on a bit about this. If you have a given pellet, it has a given mass, the same everywhere regardless of the local value for g.

    If you use a spring balance to weight it, then the measured weight will vary with g.

    If you use a balance scale, the weight you measure will be the same everywhere, regardless of the local value for g (assuming that the local value for g is the same for all parts of the balance, which I think is pretty safe). It would therefore be possible to change the scale of the balance to directly read mass rather than weight.

    So, it should be possible to measure mass independent of the value of g, using a balance scale. In fact I can envisage a means of measuring mass using a spring resonance method, which would also give mass independent of g.
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