bullets often do not travel in a straight line once they've hit something
What does that mean.... "hit something"?
Are you saying that a bullet going through a body without hitting anything other than soft tissue will not travel in a straight line?
Do you really need someone to tell you what "hit something" means? It should be self-evident, as an Illinois rail splitter might say.
What I'm saying is that you can't count on a bullet taking a straight path through a body. See this:
and this:
for example.
Note that these examples, like yours, are in ideal conditions for a straight-line path: the bullets hit the target normal to the surface and the targets are composed of a homogeneous and monolithic block of material. The structure of the human body is far from being either homogeneous or monolithic, and there's no guarantee the bullet will be travelling normal to the target's surface at impact.