What is the myth on the Magic Bullet that killed John F. Kennedy?

The theory is that the projectile hit JFK, tumbled, exited at a point of his body much lower than if it had penetrated straight through, and hit the person seated in front of him. Ballistics gel tests of the cartridge used have actually given credibility to this theory, as the Carcano round was proven to tumble upon impact, but still retain enough energy to penetrate through.