You don't need to measure MOI at butt as it is easily calculated. Therefore no need to monut the gun at butt in a MOI measuring device.
Parallel axis theorm: any axis of rotation through the CG will have a parallel axis through the butt. The MOI at butt is calculated by mass (weight/32.2) X length in feet from CG to butt (balance point to trigger + LOP qty / 12 in/ft) qty squared qty + MOI at CG.
That's pretty straightforward. It seems to me that these values would make it much easier to set up a try-gun for an initial fitting.
Another point: Thomas's apparatus measured MOI with the gun inverted on the pendulum; the rotation was around the CG in the horizontal plane. Is that value valid for all rotations, including those in the vertical plane? After all, recoil is primarily in the vertical plane.
