Moment of Inertia

Everyone has a different preference but, to me, the most important aspect of any shotgun is balance. And by balance, I don’t mean just the teeter-totter point. I mean the moment of inertia. MOI is the measurement of the effort it takes to swing the gun. It is what makes the gun lively or steady. It is a combination of the weight of the gun and how that weight is distributed.

The balance point, typically at or near the hinge pin, doesn’t really tell you that much. Here’s why. Take the example of a broomstick and two bricks. Put a brick on each end of the broomstick and pretend it is a shotgun. Wave it around. The “gun” will balance in the middle but, with the weight on the ends, it will be slow to start swinging and hard to stop. This means it has a high moment of inertia.

Now take the same broomstick and put the same two bricks together in the middle. The “gun” still balances in the center the way it did before and still weighs the same, but now the weight is in the middle and the ends are very light. It will be very quick to start swinging and very easy to stop. This is a low moment of inertia.

A third example could be made by taking the same two bricks, grinding them up and gluing them evenly all along the broomstick. The stick will still weigh the same and balance in the middle as it did before, but the swing dynamics of the stick will change yet again.

As you can see, the balance point really doesn’t tell you all that much about how a gun handles. Obviously, if the balance point is well forward, the gun will have a forward bias for its weight, but that is only part of the story.

My general rule of thumb is that the lighter the gun, the more of its weight I want forward. You need that to keep a little gun from being whippy. The heavier the gun, the more I want the weight to be in the center or occasionally a little to the rear. Of course, it all depends on what the gun will be used for. Target guns can be more weight forward to be steady, while close-cover field guns should be neutral to be fast.

Sometimes the wrong reasons produce the right answers. When I fool around with some of the older British sub-gauge side-by-sides, I usually find that they are centrally balanced like most of the 12-bore “bests.” But the sub-gauge guns are also light, because sub-gauge guns are supposed to be light. The combination of light weight and neutral balance often makes them difficult for me to shoot, as I lack the ability to control such lively guns.

Some of the very-much-less-expensive Turkish sub-gauge side-by-sides can be almost as light, but they often have much-heavier barrels. That’s because it is cheaper to make a thick heavy barrel than it is to make a light one with thin walls. The reason may be economy, but the result is that some of the Turkish side-by-side 28s and .410s are much more shootable with a welcome forward weight bias.

Naturally, we all have our own preferences as to weight and balance. Don Amos, an engineer with an automobile-tire company, is the expert on shotgun moment of inertia. He built a turntable that can measure a gun’s moment of inertia both unmounted and after it has been mounted to the shoulder. He often brings his MOI machine to the big side-by-side shoots and checks out guns for people. He has quite a list of MOIs for hundreds of different guns. It makes an interesting read to compare them. In theory, if two guns have the same moments of inertia and weigh the same, handling should be identical regardless of brand. Of course, for practical purposes, gunfit would have to be considered, too, but that is outside this discussion.

So just remember: The balance point on a shotgun is only part of the handling story. There is a lot more to it. Next time I’ll go into how to change the balance and moment of inertia of a gun.

That’s it for now. Boots off, beer open.

Don Amos Contact

Bruce,
Do you have contact information for Don Amos or information about what shows or events he will be attending and doing MOI measurements?  You may respond by email if you wish, lsmith@knotsmith.com  Thanks.