For those who reject the Single Bullet Theory, the back shot poses a huge problem. Presumably, a bullet was fired from a gun at an extremely high velocity. The average bullet travels at 1700 miles per hour or 2500 feet per second. So, if such a bullet hit JFK's back at that speed, how could it go from that velocity to zero velocity in just one inch? How could it possibly stop that fast? A tree will stop a bullet, but not in an inch. So, how could the soft tissue of JFK's back offer that much resistance? How could a projectile traveling that fast stop in such a short distance? Dr. Humes probed the back wound and said that it went about as far as the distal joint of his pinkie, which is about an inch.
Some have argued that there was a "short charge" where the muzzle velocity was much less than average, perhaps as low as 760 feet per second. But, even at that speed, going through soft tissue, surely it would travel farther than an inch. And on what basis can we assume a "short charge?" There is no evidence for it, and mathematically, the odds of it happening are extremely remote.
Then, there is the theory that it was a sabot round, which refers to the device that is put around a smaller caliber bullet, so that it will fill a larger caliber muzzle. The sabot then falls off. But, much more often the term "sabot" is used to describe armor-piercing rounds that are shot at tanks. But, I know of no good reason to assume that JFK was shot in the back with a sabotted round. Why even go there?
The most reasonable thing is that he was hit with a projectile that was designed to burst. You've probably had the experience of an ice cube bursting when you go to pry it out of its compartment in the tray. That happens when the ice tray becomes coated with hard-water mineral deposits, particularly calcium carbonate. If the projectile was made of ice, it could have been deliberately formulated to shatter, which is to say explode.
Mythbusters did a segment on ice bullets, and they particularly referenced the JFK assassination, and they quickly got to the problem of ice's brittleness. But, they must have known about the Church hearing, and they actually thought that the CIA would enlighten them on how to make it work. So, they wrote to the CIA for information about ice bullets and ice guns, and what they got back was a letter in legal-ese saying that the CIA is exempt from the Freedom of Information Act and would provide no information about it. And even the Mythbusters guy said, "Well, that seems to be saying that there is almost definitely something like this in existence."
But, they proceeded with their tests, using instead of ice, frozen gelatin, and you probably remember, as I do from childhood, that making JELLO involves using a lot of water. And they had a test dummy with similar density as a real human. And the result was that the gelatin bullet penetrated the dummy about an inch- the same depth reported by Dr. Humes.
Remember that at the Church hearing, they had the gun; they were all peering at it; and they should have tested it, but they didn't. But, I assume that the gun worked. Don't you?
The point is that there is a problem in realizing that JFK was hit with a projectile that struck him with tremendous force but was immediately stopped, that is, within an inch of penetration. That has to be explained. The idea of an exploding ice round is more plausible than either the short charge idea or the sabot round. Newton's laws of motion deny that a solid, intact bullet could have been stopped that fast. By what? JFK's soft issue? That's ridiculous. It's time to start taking this ice bullet idea very seriously.