Problem is, builder, for all presentations you don't pivot at the waist. How about a dropping bird, do you bend down at the waist as the bird drops? I don't. Fact is, for most people, if you track the target far enough to really need to pivot at the waist, you're not shooting properly. And then there is that brief time when you are mounting the gun and acquiring the target at the same time. When you can do this properly you can shoot the instant the cheek "welds" to the gun. Huge differences in how well you can do this with guns of differing MOI.
John came as close to hitting the nail on the head as you can. Balance means nothing, if balance is described as where the gun balances under the receiver. You can have a 6 1/2# gun that is so "quick" that you may over-correct and be waving it around trying to settle on a straight away bird, because the weight (mass) is highly concentrated in the receiver of the gun and very little is on the ends (butt and muzzle). OTOH, you can have a 6 1/2# gun that has the weight concentrated on the muzzles and butt, and a lightweight alloy receiver, that feels almost "sluggish". This phenomenon is MOI, moment of inertia.
I have a little 28" .410 S x S that has an alloy receiver. The barrels were not struck very thin towards the muzzles. and the buttstock is full sized and fairly dense walnut. Rocketman spun it a couple years ago on his machine and found that it's MOI is almost exactly that of a 12 g. English game gun, which has the weight concentrated exactly opposite. I shoot the little gun very well for something that only weighs 4# 13 oz., but it has nothing to do with the balance. It has everything to do with the moment of inertia.
I hope no one believes that all guns that "balance on the hinge pin" handle the same.
SRH
