Ok, I'm pressed for time today, but here's a short version.
High velocity rounds tumble after a certain amount of distance in flight due to not being perfectly aerodynamic after being fired. Can't help it.
Additionally, while human flesh is soft and easy to penetrate, things like bones can radically alter a trajectory if struck. A bone can't negate much of the energy left in the round, but it can change the path.
Listen, I've seen high velocity rounds do some fucked up things. The "magic bullet" theory isn't so unbelievable.
Here's another warning for ya: James Garrett, played by Costner in the movie, was a fucking psycho. Do some research on his background...he really was a conspiracy nut who tried to prosecute everyone. The whole JFK movie is a joke.