doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com MOI machine calibration and demonstration

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

Hopefully this will do something about those "it balances half inch in front of the hinge" type of analyses. Where it balances has no bearing on the handling, as Rocketman has proved yet again.

Rocketman,
Nice illustrations.
Are you coming to Southern SxS this weekend ?

In actual use, where is a gun's pivot point? Is it at the point of balance or somewhere else? Doesn't the actual pivot point depend upon the geometry of the gun in relation to the shooter?

If the MOI is calculated using the gun's balance point and the shooter does not pivot the gun around that point how do the measurements relate to the actual experience of shooting the gun?

Why don't you ask your hero, Barack Hussein Obama, what he uses as pivot and balance points when he is posing with a shotgun for propaganda images in order to advance the gun control agenda you told us he would never attempt?

I acquired a 12 bore British hammergun that is just over 8 lbs. The balance point is about 5 1/2 inches in front of the front trigger which in my mind makes it feel even heavier than it is. I considered adding some weight to the butt and used some masking tape to do so experimentally just to get an idea of the difference in feel. I added about 8 ounces which moved the balance back about an inch and it did make the gun "feel" lighter even though I added weight to it. Then I did some reading and the opinion was that adding weight to either end of a gun to change balance is the worst place to put it because of the MOI. I am curious about how people are putting the information regarding MOI to use. How does changing the balancing point on a gun, in this case moving it back while also adding weight, change the MOI?

Do you get the center of mass over the pivot? I fully understand MOI, but I still believe the balance point has a heck of a lot to do with how a gun feels to an individual.

I think you're right - I cannot visualize where the gun pivots when I shoot but I'm going shooting tomorrow and will try to figure it out on the field. I suspect the closer the center of mass is to the actual pivot point when you're shooting the gun, the better it will "feel"

Let's say that in order to move back the Center of Gravity by 1 inch you need to add 4Oz at the butt, you could also add 8 Oz 1/2 way in the stock. Because the moment of inertia is proportional to Mass x distance squared the second solution has 1/2 the additional inertia. The penalty is that it also has more mass.
e.g. there is no free lunch.
Balancing should be static and dynamic and it is quite an art (that I know nothing about) on complex objects such as shoulder guns.
Attempting to measure things is a necessary first step in learning about this subject matter.
Best regards,
WC-