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.

If anyone doesn't get this or wants to see a sample proble, post back.

DDA