There is no evidence that a bullet struck JBC even close to Z271. That is the result of your FUBAR figuring.
That is simply an incorrect statement. There is evidence (shot pattern, Altgens, Hickey Nellie, Greer, etc). You just refuse to accept it. Do you really think the FBI went to the trouble building all these models without some evidence:



In order to believe your scenario, I would have to believe that both JFK and JBC were hit by Oswald's first shot at a time both would have been obscured from Oswald's view by the pin oak tree in front of the TSBD.
JFK was visible when he passed the lamp post. You obviously have trouble recalling what I have posted many times:

I would have to believe that neither reacted at that time to being hit. I would have to believe that JFK began to slowly lower his right arm and that neither man showed any reaction for 33 frames, almost 2 seconds, and then suddenly both flipped their arms upward rapidly at the same instant (Z226).
JFK is obviously reacting before z225 because he is already in a contorted position. Look at his face and hands.

I would have to believe that JBC after rapidly flipping his right arm up and down then dipped hard to his right, doubled over, then began twisting around dramatically until he was facing JFK at Z265. All of this movement was a reaction to his shallow wound in his left leg.
You are apparently not familiar with what he said he did in reaction to the first shot. What he did from z230-270 is generally consistent with what both he and Nellie he said he did after the first shot-he turned around to his right and uttered “oh, no, no” because he knew he had just heard a rifle shot and thought assassination was underway. Jackie turns to look at him, as she said she did, when he starts shouting in the mid z240s. Nellie said he uttered that before the second shot, which fits. JBC said it was as he was hit, although he said it might have been before he was hit (to the HSCA).
Now comes the really weird part. You expect us to believe that Oswald fired a shot at Z271 that hit JBC in the back, exited from his chest, and struck his wrist at frame Z271. This frame Z271:
https://assassinationresearch.com/zfilm/z271.jpg
Just how in the hell does Oswald shoot JBC in the back at Z271 and have the bullet exit from the right side of his chest and go on to fracture his wrist?
It just requires a slight deflection to the right on the bullet hitting the back.
If that isn't unbelievable enough, you then have Oswald firing a precision headshot just 42 frames later at the bare minimum time a shooter could theoretically have done so. Am I really supposed to believe all that?
No. That is just what the evidence says happened. You don’t have to accept it.