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#289648 08/18/12 03:42 PM
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xausa Offline OP
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In his "Gun Book" British author Gough Thomas describes an imperical test he used to define the "balance" of a shotgun, by which he meant a traditional British side by side. He said that the measurements he made included weight, point of static balance, least transverse moment of inertia and radius of gyration.

The purpose of the test was to quantify the amount of effort required to swing the shotgun in question.

He included a photo of the device he used, which appears to be a cradle for holding a shotgun inverted while suspended from above by what appears to be a thin rod or wire.

He offered no explanation as to how this measuring device was used, although there is a stop watch included in the photo, so evidently accurately measuring time is part of the procedure.

Can anyone shed light on what the measurements consisted of, how they were arrived at, and how the figures obtained are to be used?

xausa #289690 08/18/12 10:46 PM
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Rocketman, where are you?


So many guns, so little time!
xausa #289694 08/18/12 11:15 PM
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Basically, the whole contraption pivots around the suspension wire. The operator rotates the pendulum a small amount, say 10 deg., and starts it oscillating. The period "T" of the oscillation is established and plugged into the moment of inertia formula. The formula used for a two wire pendulum is I=(M*g*T^2*b^2)/(4*pi^2*L), where I=moment of inertia, M = mass (kg), g = acceleration of gravity (9.81 m/sec^2), T = period of oscillation (sec), b = distance from suspension wire to center of gravity (m), L = length of suspension wires (m).

I believe, that since T^2 and M are the only variables in the equation, one could simply make that calculation and be able to directly compare different guns. Moment of Inertia, I, is an indication of the energy required to change the orientation of an object; so a low figure would indicate a "lively" gun. A high figure would indicate just the opposite.

Last edited by Carpetsahib; 08/18/12 11:16 PM.
xausa #289696 08/18/12 11:54 PM
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Ah physics. I think I'll just stick to shooting the gun and see how it feels. My cousin is a engineer and he loves this kind of stuff. Quantitative analysis is his thing. I am a medical professional and have a much more philosophical/abstract mind.

James

xausa #289698 08/19/12 12:29 AM
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Here I yam, Builder!

xausa, I have been providing this measure for some 12 years now, although with a differing measuring device. I use a constant torque turntable with one revolution timed from start to completion. My current database is over 600 guns with gun description, weight, balance (teeter-totter point balance = CG, not some magical feel sort of thing), unmounted swing effort (MOI at CG), mounted swing effort (MOI at butt), and half weight radius (compactness). The turntable is kept calibrated via metal bars of calculated MOI; several differing MOI's are used to make a chart with a regression line (R squares are very high indicating a high degree of accuracy).

Thomas's device was based on a torsional pendulem. It is sufficiently accurate, but does not lend itself to "production" of data. My data fits well with Thomas's.

What more would you like to know?

DDA

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xausa Offline OP
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Rocketman,

Thank you for your response, although as someone who barely made it through calculus 55 years ago, I don't pretend to understand the details of what you and Carpetsahib are laying out, except in the broadest terms.

However, my original inquiry was just a lead-in to a more basic question, which is a request for a comparison between what I have been led to believe are the top level American made doubles, such as Fox, Lefever, Parker, Ithaca and the Winchester Model 21 as opposed to British and Continental "best" doubles, in terms not only of "balance", but also of stock design, action strength, durability and robustness, and sophistication of locks in terms of ease of cocking, resistance to doubling, weight of trigger pull, and lock time.

Since my experience over the years has been primarily with over and under shotguns, starting with Brownings and progressing through Perazzis to the SO series Berettas, I have very little first hand experience with side by side guns and would greatly appreciate whatever crumbs from your table of expertese you would care to brush off in my direction.

xausa #289748 08/19/12 02:22 PM
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Rocketman, will you be at the Vintage Cup in Rhode Island this year?


So many guns, so little time!
builder #289750 08/19/12 03:31 PM
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Builder, I working on a viable plan to do just thst. Will let you know.

DDA

xausa #289755 08/19/12 03:37 PM
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xausa, next "pearl of wisdom."

I assume you understand stock measurements and would agree that they can't be "summed up" by any one measurement or descriptor. Consider that handling dynamicas are the same in that it takes four measurements to describe any particular gun's handling. Also, consider that, just like stock dimensions, handling dimensions need to be "fitted" to the individual shooter.

All for the moment. Let me know if the above "computes" for you

DDA

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This is highly interesting. How do you mount a gun to determine the MOI at the butt?

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