I had one that really was a mystery to me.I had done three stocks for a shot gunshooter and all worked well for him. Then the last one threw me for fits.Turns out the front bead was higher than normal and it was throwing himoff. We replced the bead and all was well.

Normaly the same stype of shotgun will have the sam ebasic measurements for the shooter,but styles will shot differnt and O/U will be somewhat different than a SXS.

I have Fox Steringworth and an a NID Ithaca that I shoot and each has slight variations in the stock.
Cabelas sells a strap on device with shims that you can add or subtract to vary the comb height to get what you need. Greatway to start off.
Several shotguns now have shims that you can add to the raer of the reciver that varies the angle, ie changes the drop and cast.