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

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.


[img:center] [img:center][img:center][/img] [/img][/img]






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.


[img:center][img:center][/img] [/img]

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.

[img:center][/img]


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!)

[img:center][/img]

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

What shows will you be at to take measurements this year?

Most likely the Southern's and maybe the VC. Also possible at Tulsa in a couple of weeks. Love to work with you.

DDA

Like the little Yildiz .410 S x S with the alloy receiver.

Well explained, Don, once again. This was a revelation when Don helped me understand it. I had been a shotgunner all my life, and tossed around words like "balance" and "feel" and "swing" to attempt to describe the differences in shotguns, but when I spent a little time with Don and he showed this principle to me it was literally an epiphany. It is so nice to really understand the dynamics of a gun, and why they feel like they do.

We owe Don a huge thank you for bringing this to the forefront by building this MOI measuring device and spending so much of his time measuring our guns, to say nothing of the database he is compiling. It alone could help tremendously in determining how a gun will handle for us, before purchasing.

SRH

I still don't understand. Would love to see it in action at Tulsa. I'll be hanging around Champlin Firearms on the upper level.

We really enjoyed having Rocketman and his lovely bride at Flatwater a few years back. He made everybody well aware of this by being able to explain in kindergarten terms.

Joe, I hope you get to see it in action and talk with Don. It is time well spent, and fun!

Here is my way of explaining what Don told. My little Yildiz has an aluminum alloy receiver, which is lightweight. The fore-end is very small (splinter), so not much weight there either, in the middle of the gun. It has 28", fairly heavy, barrels, and a full man-sized buttstock of fairly dense walnut, both of which add weight to the "ends" of the gun. So, even though the gun may weigh exactly the same as a gun that has a steel receiver, shorter lightweight barrels, and possibly a lighter weight buttstock, the weight that is there is in different places, i.e., out toward the ends of the gun and not in the middle. It's a weight distribution issue.

By more of the existing weight being out on the ends it makes the gun just a little "slower" to swing, or put another way, it requires more swing effort. That makes a lightweight gun "swing" like a heavier gun. The MOI of that little 4.8 lb. gun, according to Don, is almost identical to the MOI of a classic 12 gauge British 6+ lb. game gun. Hard to believe, I know, but it helped me understand why I can shoot the tiny little thing halfway decent, when most of my shooting is with a 9# Perazzi.

SRH

Don - we are all indebted to you for bringing little objective analysis to what is a stubbornly subjective pursit.

Here is another way to think of it in practical terms. Think of the military training exercise where guys beat on each other with a pole which they hold in the middle and which has pads on each end. Do you want the pole's weight on the ends or between your hands? If the pole's weight is distributed towards the ends, it will have a high MOI and you won't be very quick attacking or defending yourself. (I think I got that right).

Don, I still don't understand how the gun is spun and how you get your MOI reading. Doverham, the padded sticks are called "pugil sticks" and are the most draining of any training exercise in Army basic training.

Don,
The LACA will have a bunch of Lefevers at the Southern this year. We hope you can stop by and add some to your data base. I will be interested in how they compare to other American and European classics of that era.