An article in ‘Guns’ showed a cut-away Soviet 5.45 bullet. The lead core was designed to do just that. They were impressed with the tumbling characteristic of our M-16 round.
The same author wrote, if there was a way for the Russians to make bullets from concrete, they would have done it!
Fackler notes the temporary cavity doesn’t do much damage to anything except the liver, brain and spleen. The rest of the body is pretty flexible, except bones which are pretty rigid under hydrostatic loads insufficient to make a permanent cavity.