Bill,
There is a little weight that hangs off the edge of the table on a cord attached to the horizontal wheel. Don times one revolution of the wheel, several times, then averages the times, as I recall. This elapsed time is used to calculate the MOI.
SRH
Stan, a good explaination. I'm going to fill in a few commonly asked questions. The turntable is bearing mounted to a base plate. A string is wound on the outter perimeter of the turntable. The string runs forward over a turn-down pulley and is attached to a small bob-weight. This arrangement provices constant torque to the turntable. The turntable has clamp blocks mounted so as to allow "fixing" a gun (mounted horizontally and upsiode down) onto the top of the turntable. The turntable is kept from rotating until ready by a spring powered brake/trigger. The trigger contacts the edge of the turntable and is also in contact with a stop-screw set into the turntable on a radius. When the trigger tail is pushed, the upper arm loses contact with the turntable rim and with the stop-screw. The turntable immediately begins to rotate under the constant torque of the falling bob-weight. The first movement of the table trips a magnetic reed switch which triggers a stopwatch. One full rotation is allowed. As the matnet passes the reed switch the second time, it triggers the stop function of the watch; we have the time for exactly one revolution.
Moment of inertia can be easily calculated for bars of constant section: round, square, rectangular, etc. The machine rotation times can be calibrated by use of a series of differing bars of assorted calculated MOI. The various times vs known MOI are plotted into a chart that allows accurate conversion of time to MOI.
Per Stan's example above, the Churchill has the following handling "fingerprint:"Churchill 25 BLE #4614 weight = 4# 1 oz, balance to front trigger = 3 3/4", unmounted swing effort (MOI @ CG) = 0.84, mounted swing effort (MOI @ butt) = 3.74, gauge = 0.410, bbl = 25", LOP = 14 1/2" and half weight radius (measure of compacthess) = 9.82.
Yildiz Elegante BLNE #7140 weight = 4# 14 oz, bal = 5 3/8", MOI@CG = 1.48, MOI@butt = 5.60, gauge = .410, bbls = 27 7/8", LOP = 14 3/8", and HWR = 11.83.
The Yildiz is 21% heavier, has its teeter-totter point 1 5/8" farther forward, requires 76% more effort to swing before mounting, and 50% more effort to swing when mounted. All that increase in swing effort for only 13 oz???? Yes, definitely. The 20% increase in half weight radius tells us that the Yildiz is much less compact than the Churchill.
Well, Joe-in-Charlotte will tell you the Yildiz is a numb club and Stan will tell you the Churchill is an unruly wand. Who is right? Both --- for themselves. Joe and Stan can now look at a gun handling "fingerprint" and tell if the gun will "fit" their handling preferences just as they can look at a set of stock dimensions and tell if the gun will physically position their shooting eyes in the right place.
Questions?
Dr. Bob, I'll see you in Sanford. Joe Wood, I think I'll be at Tulsa, but the MOI machine is in question; at least we can have a good 'ole jaw session.
DDA
