Ray, when you said hogwash, did you mean physics 101? If your knowledge were as big as your bluster, I would not have to keep taking grains of salt as I read these posts.

The mercury remains immobile while the rifle begins to move under recoil.

Then the end of the mercury tube slams into the stationary mercury, retarding the recoil of the rifle.

Lead in the butt begins to move at the same time as the rest of the rifle and does not provide the counterforce of stationary mercury. Hence the greater recoil reduction of mercury.