doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Question about MOI; creating two guns

I wish to create a gun for myself that feels like a friend's gun which I shoot very well. Both are Italian target O/Us. Both have the same barrel and overall gun lengths. Mine weighs 9 oz. less and I wish to have a heavier gun anyway.

I understand that MOI takes into account balance point, overall weight and swing weight. Buy adding weight fore and aft, I can get my gun to balance and weigh the same as his.
However, I understand that the distribution of that weight will affect the MOI of the gun.

I do not have access to a MOI machine.

How do I go about determining where to distribute the extra weight to "match" my gun to his?

The most accurate answer would be a very dense mass added at a precise radius from the center of rotation. But, having done this exercise in the past, I would avoid any math altogether. I used a gold slug placed at a specific distance from the hinge pin, within the buttstock. I couldn't get any depleted uranium.

The easy answer for you is to get a scale, and then begin to weigh and compare the main elements of the shotguns.
Barrel Monoblock assemblies, choke tubes, action assemblies, trigger groups (if drop out type), buttstock, forend assembly, and recoil pads.
You will probably find your gun differs from his in wood density. With most of the production components being very close in weight.
If the difference is in the barrel monoblock asm sans tubes, it is quite difficult to lighten the assembly. Expensive too.
Very likely, the differences are a heavier forend, or a denser buttstock. You can add/subtract mass pretty easily from either. Their similar lengths between the shotguns narrows the effect the radius term will have after the mass is added.

I'll bet dollars to doughnuts that after the Vintager weekend, Don will post the formula, and his methods again, and you'll see how moving a mass away, or toward the desired point of rotation effects MOI measurement.
As I recall, in my earlier experiments with an adjustable MOI aparatus, the denser the mass, the less it needed to be moved to change MOI. Mass had more effect than radius.
Realistically, you will probably work on the forend asm wood, and the buttstock wood, as all the other pieces are probably very similar in length and mass.
edited to ad: Choke tubes, being a great radius away from the center of rotation can have a big effect on moi. Titanium vs steel etc. Also, Don's methods include dynamic efforts as well, not just static ones. 2 guns can weigh the same, but have vastly different MOI measurements.
But if you can get the average density distribution of your gun similar to your friend's, it will feel substantially similar. But you are limited in where and how you can make changes.

Thanks ClapperZapper!
This is the information I am looking for. I agree that overall weight and balance point is only part of the equation. I need to figure out where to place the additional weight. I agree with your idea about carefully weighing and balancing each of the components on both guns to see where differences lie. There are various ways to add weight here and there as needed. I anxiously await hearing from Don too.

I was forced into the situation I described because I wanted an O/U with a straight hand stock.
I was an idiot, and listened to some moron, perhaps he was only an imbecile as I think about it, who said you can cut a pistol grip stock and end up with a satisfactorily balanced straight hand shotgun.
You can't. The buttstock has too much material removed to balance the shotgun when first stocked.
Cutting 8-9oz of walnut from a pistol grip stock shoves the balance about 4 inches past the hinge pin.
My efforts were to repair my ignorant mistake.