Why, of course I fill the cavities of my hollowpoints with Plutonium Nitrate. Ya see, the plutonium is crushed beyond critical mass at the moment of impact, and the lead acts as a reflector for neutrons, with the result there is a teeny tiny little nuclear explosion, creating a wound channel the size of a grapefruit. However, the burst of gamma radiation emitted kills everything within 21 feet instantly. We used them all the time when I was a SealRangerSniperScoutRecondoDeltaAirCommandoSAS Operator for the CIA.
Ladies and gentlemen- if there was a more effective means of making a bullet work, do you THINK it just MIGHT be possible that one of the commercial ammo makers that spends a LOT of money on R&D would be doing it?
You might also look back to the Hoxie Ammunition Company of Chicago from 1907 to the '30s. Their bullets were characterized by a small steel ball embedded in the tip to aid in expansion on contact.
In the same era there were large bore rifle bullets that held a .22 blank inserted into the cavity- to fire on impact.
None of them worked especially well.