I needed a better calibration device for the MOI machines, so I did a little "building." This is the prototype mounted on my MOI machine. As I worked with it I realized it was a very good demonstration of the most difficult part of moment of inerta to understand. As the weight moves away from the center of rotation the MOI increases as a square of the radius (like a rifle bullet increases kinetic energy as a square of velocity).
The first photo shows the weight boxes/slides at their smallest radius, 3 1/2" from the turntable pivot. This setting gives a MOI of 0.37, a MOI that is too low for a practical gun, but important to machine calibration.
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The second image shows the weight slides set at 6 1/2" of radius. The MOI is 0.81 which is very similar to a Churchill XXV BLE .410 (0.84)in the database.
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The third image is with the weights set at 9 1/2" of radius. The MOI is 1.45 which is typical of a 6 1/2# Brit game gun.
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The fourth image shows the weights set at 12 1/2" of radius. The MOI is 2.31 which is typical of guns similar to the Browning BT-99 or Beretta 687EELL 12 bore with 29 1/2" bbls and 14 3/4" LOP.
The fifth image is with the weights set at 15 1/2" radius. The MOI is 3.92 is in the range of 12# 8 bore 32" bbl SXS guns (extreme fowling!)
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I hope this will show more shooters how quickly MOI (swing effort) builds up with increasing weight away from the balance point of the gun (minimum swing effort occurs when the gun is swung about the balance point). Also, note that the weight and balance point of the cal. stick did not change throughtout this test. This shows that MOI is independent of weight and balance; a light gun can have higher MOI if the weight is moved far enough (near muzzle and butt) from the balance point and vice-versa.
Questions?
DDA
