It’s a pretty simple weighted average calculation.

Recognizing that the barrels weigh several times what the rear half of the stock weighs.

Unless they are very short.

So, once a person cuts the pistol grip off which is behind the center of balance, it magnifies the barrel heaviness.

You end up adding a pretty heavy piece of lead in the back to counteract the concentration of mass in the barrels.

Out there doing it best I can.