Monday, March 11, 2013

IT'S IN THE NUMBERS


A mathematical look at why some shotguns handle better than others.

How a shotgun handles is a subject of infinite debate. There are those who like a very fast gun, and those who favor a shotgun with  more weight  forward. It's all subjective, but some thought is often necessary to separate out the gun that whips around at imaginary birds in a gun shop and one that performs really well with live birds in the field.
The late Gough Thomas was the dean of British shotgun writers in the middle decades of the twentieth century. In a compilation of the best of his writings, one chapter is tided "The Importance of Balance." There he says, "The active manipulation of a gun involves considerable expenditure of energy, and it can be shown that less perhaps far less-energy is required to manipulate a well-balanced gun than is involved in putting a badly balanced one through the same motions."
Thomas continues, "Good balance requires that the weight of the gun shall be concentrated as much as possible between the hands . . . it will in consequence feel responsive, eager, and cooperative in action . . . but if the gun has its weight more dispersed fore and aft, it will feel leaden and sluggish."
What does this all mean? Let's take a fine British-made double that has the weight between the hands and give it, as Thomas d id, a "figure of inertia" of 100, representing the energy required to swing on a bi rd. When compared with another, less well balanced, shot­gun with a figure of inertia at 150, it is clear that our less than excellent shotgun will require half again as much effort to shoot the exact same target.
Thomas was an engineer by trade, and reduced the subjective "feel" of a shotgun to mathematics that provides direct comparisons of guns. As chance would have it, I met Don Amos, who is also an engineer and fine shotgun aficionado, at the Las Vegas Fine Arms Show last January. He was testing shotguns with an interesting-looking little machine t hat spun them around. (Thomas used a cradle that swung like a pendulum.)
When queried, Amos responded that his device was testing for moment of inertia (MOl). If you look up moment of inertia  you will get several pages of engineering discourse, but in a few words, it means the amount of energy required to start an object moving in one direction. For the shotgun nee it does not mean the effort to bring the gun to the shoulder although a good gun mount incorporates both the lifting of the gun and the swing of the gun but the effort to swing the gun laterally on a target.
If our theoretical shotgun's moment of inertia is low, it will take less effort by the shooter to swing the gun, and hence less drain on the body, making it less tiring and making our shooter a better shot at day's end. When I expressed keen interest in Amos's proj­ect, he agreed to make me a similar machine so I could test various shotguns for my own interest.
The machine is calibrated so that when a gun is put on its rotating top, the amount of time it takes for the gun co complete a full circle is measured on an attached stopwatch.That, along with the measurements of the gun barrel length, length of pull, weight, and most important, the center of gravity or bal­ ance point are fed into Amos's Excel program (results can also be calculated mathematically) and  it  provides  the measure of the moment of inertia.
The resulting figure is not a measure of foot-pounds or any other directly relatable measure, but rather a comparative number that can then be used to define the handling characteristics of a given shotgun when compared to an­ other. For example, Amos directed me to a $20,000 used Purdey and a $3,000 Spanish-made gun. Both  had similar MOis, and I found they were very similar in the way they felt and handled, price difference notwithstanding.
For example, the MOI of my wife's Beretta Silver Hawk  is 1.36; my 6pound AyA No. 2 28-gauge is close-by at 1.16, and so too were a Benelli Super Black Eagle II at 1.92 and Beretta's new A400 at 1.96. Moving to heavier target guns, an 8-plus-pound Perazzi MX8 has a MOl of 2.27 and a 30inch barreled Beretta DT 10 has a MOl of 2.32.
Another facet of Amos's calculation has to do with what he calls the Half Weight Radius (HWR), which defines how the weight is distributed across the balance point of the gun. Recall Thomas's solemn admonition regarding the weight being farther from the balance causing the gun to become sluggish. The  HWR of the Silver Hawk is 9.97, meaning that the majority of the weight falls about five inches on each side of the balance point, which  is 5% inches ahead  of the trigger. The AyA's balance point or center of gravity (CG) is 3% inches in front of the trigger with a HWR of 9.73. The Perazzi and DT 10 ended up at 11.34 and 11.38 respectively, with the DT lO's CG  nearly an inch far­ ther forward. That difference makes the  Perazzi  feel  more  barrel-heavy while the DT 10 feels more balanced between the hands, yet both are dose in MOl and weight distribution.
What does this prove? Some could say not much, because both beauty and gun  handling are mostly in the eyes and hands of the beholder. However, it demonstrates why guns feel the way they do, and gives us a better under­ standing of the intricacies involved in the formerly nebulous mystery of how a gun handles.


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