I did the conversions of kinetic energy to momentum as a function of a for-sake-of-argument mass of the grain of sand and a zero-elasticity collision.
I will repeat my latest for-sake-of-argument figures: Grain of sand has mass of .1 milligram. The 4-foot-diameter ball has mass of 2.5 metric tons.
When floating in a vacuum, if hit with a dead-center hit, the above big ball has velocity changed by close enough to 1/25,000,000,000 of the velocity of the grain of sand. (At least in terms of vector subtraction result's magnitude, where the vector subtraction is between the big ball's velocity before the collision and the big ball's velocity after the colision - regardless of direction that the big ball was shot from.) Between that much velocity change and twice that if the grain of sand bounces back. Add to this if a crater is created, due to expelling the mass of what was where the crater afterwards is (as in mass that was expelled. Net momentum of mass expelled from the big ball means a similar magnitude opposite direction momentum was added to the big ball, in addition to that which the bullet had, minus any [by vector subtraction] that the bullet ricochets with if it does). For that matter, rubber bullets actually kick harder than do clay ones of same mass and velocity and similar size and density. The rubber ones continue their "kick" by pushing off their targets for ricocheting, in addition to the impulse required to stop them.
I do agree that extremely little of the energy expended by using a high velocity particle or an energetic vaporizing explosive detonation becomes increase of kinetic energy of a massive target. However, I think the problem here is more one of changing the target's momentum than one of changing its kinetic energy. It appears to me that it gets down to energy-efficient (or more like cost-efficient) ways of hitting a massive target with an "impulse" - which is product of force and time, or more exactly integral of force(as_function_of_t)*dt over the period covering the t / time when the force is applied.