What are the two or three best artificial media for testing stability of a bullet after impact? By "stability," I refer to a bullet yawing in a different direction from which you aimed it. The best example of instability - that was planned for during development - is the original 5.56 mm military bullet to be fired from an M-16 with 1:12-inch rate of twist. Upon impact with people, the every-which-way that bullet traveled wreaked tremendous damage - but didn't penetrate well. Current 5.56 mm ammunition-rifle unit now penetrates well but is much less effective in terms of destruction on human beings.
