When the shock wave strikes the border zone of another medium or another body, reflections are created along this border zone. The scope of these reflections depends on the acoustic impedance of the two bodies (media).
The angle of impact on the border zone is decisive as well. Acoustic impedance in metal is much smaller than in water so that the shock energy is reflected to a high degree at the metal surface.
As a consequence, the shock waves traverse the inner part of the casting several times due to multiple reflections.