This link provides a very good summary of what we know about the CIA poison dart gun:
So, the gun was practically silent, and the impact was practically undetectable to the victim. And that makes sense in this case because JFK was shot in the back high on the hill, and if he had reacted to being shot in the back with a bullet, as anyone would, all would have seen it. But, the only thing that Jackie noticed was he started to look "quizzical" and that was from whatever twitch he felt and the rapidly occurring effects of the drug or drugs that were delivered.
Some have tried to say that the purpose of the back shot was to implant a Carcano bullet in his back- as evidence. But, that is ridiculous because there was no bullet. There was no bullet in his back or anywhere else. Some have tried to say it was dug out of his back at the "pre-autopsy" but if the whole purpose was to put it there, then why take it out? But, we don't have to consider that because nobody took any bullet out of his back, and it certainly didn't fall out. The whole story gets more and more ridiculous the more you look at it because the bullet had to have enough speed to get to him, which means that it would have traveled much farther inside of him. And if you look at the wound, you can see that nobody messed with it. It looks clean. If there was a bullet in him, it would have been necessary to dilate the opening, then get surgical pliers in there to firmly grasp the bullet, and then pull it out, with the arms of the pliers and the bullet coming out at once, which is a wider diameter than the bullet alone. You couldn't take it out as cleanly as it went in.
The only thing that makes sense is that he was hit with that CIA frozen dart gun which delivered a drug or drugs which caused the two abnormalities we see in him post-shot: dyskinesia and disorientation. JFK is out of it mentally in the Zapruder film. He's like a zombie or a drunk, and that cannot be attributed to the physical damage from the back shot or the throat shot.
The back shot definitely came first. It came before Jackie noticed him looking quizzical. It occurred slightly before the Croft photo. The throat shot occurred when he was behind the sign in relation to Zapruder. But, the sign did not block Zapruder's camera as much as it does in the Zapruder film. They definitely beefed up the sign.
For the back shot, they were NOT trying to implant a bullet in his back for show. They had no need to do that. They had no means to do that. And they obviously didn't do it since there was no bullet there.
The purpose of the back shot was to create the effect in Kennedy that we see in Zapruder 225 to 313, which he is unable to control his hyper-spasmodic muscles and he is mentally torpid. Those were aberrant behaviors that had to be caused by something, and what caused them was the back shot, not the physical trauma from it, but rather, the drugs that were contained in it.
I just read today that the FDA is now saying that sun block compounds that you apply to your skin as sun screen are absorbed into your blood. So, if a chemical can go from the surface of your skin into your blood, just from rubbing it on and not traumatizing the skin at all, then bursting a frozen projectile into the back under the skin can surely get compounds into the blood.
The throat shot surely was not taken from the Triple Underpass. That shot went EXACTLY where they wanted it to go: into his throat, his voice box, incapacitating his ability to speak. It was not meant to go through his neck. It was not a kill shot. There was nothing accidental or arbitrary about any aspect of that shot. If it was taken from the Triple Underpass, think about the momentum that the bullet would have needed just to reach him. How could they possibly get it to stop in a fraction of an inch? How would they know that it would? When you think about how superficial it was, there is no way that it was shot from that far away.
I think it's likely that they were planning to use the Single Bullet Theory all along. They wanted to get that dart in his back to deliver the drug or drugs, but how were they going to explain it? How were they going to claim that Oswald's rifle did that?
And think about this: they only wanted to kill Kennedy. So, why would they take the chance of shooting him in the back with a high-powered rifle with a full metal jacket bullet that could easily traverse him? Connally was LBJ's protege'. They certainly didn't want to shoot him. So, that back shot was definitely just the dart that was sure to go nowhere; to just lodge in him and start dispersing the chemicals. But still, it was going to leave a hole in his back which had to be explained and related to Oswald's gun and ammunition. So, they asked themselves, what would Oswald's gun and ammunition do? The answer: probably go through him. So, by making that hole in his throat, they not only silenced him; they created an exit wound for the shot to his back. They knew they were going to be in control of the autopsy.
So, when Arlen Specter came up with his single bullet theory, you've got to think of him as just the spokesperson, the pitch man for it. He got the idea from the FBI.
And think about this: They probably had the expectation that JFK would be dead on arrival at the hospital. And why not? They were going to shoot him in the head, and most people die instantly or within seconds when they're shot in the head. I don't think they expected him to arrive at the hospital alive. They expected him to be resoundingly dead by the time he got there and declared so immediately. And as soon as he was declared dead by Dallas doctors, they would have said, "Thanks; now you're done." They wouldn't have let them do any poking around.
So, I think they set up that shallow shot to the back and the shallow shot to the throat in order to create the option of claiming that the Carcano bullet from the high-powered rifle traversed him.
And think about what the FBI first said about the shots, that there were 3, that one struck Kennedy in the back; one struck Connally in the torso; and then there was the fatal head shot to Kennedy. After the injury to Tague was reported, there had to be a bullet for him, so they combined the shot to Kennedy's back with the shooting of Connally. But, what would they have done if Tague hadn't been scratched? They still had two wounds on JFK's body (back and neck) which had to be explained by one bullet. My point is that: even without the Single Bullet Theory as we know it, they would still have had to argue for a single bullet causing two wounds on Kennedy. There was still going to be a Single Bullet Theory for the back and neck wounds on Kennnedy.
So, that settles it: the Single Bullet traversing Kennedy was baked into the cake from the beginning. They could never explain an FMJ that only went into him an inch and then disappeared. So, they knew ahead of time that they had to say that it went through him. So, they had to shoot him in the neck to create the exit wound for it. Obviously, the location of the wounds was very important, as was the depth of each. And since traversing him was the story they were going to go with, it meant that there couldn't be a bullet left in him.
So, that means that the neck shot, like the back shot, had to be bulletless, using just the frozen dart that dissolved and vanished. I don't know if the neck shot also contained a toxin, but it might have.