Marjan, I have actually been working on some of these measurements and calculations for a long time. I actually did extensive work several years ago and I am just now finding time to work on it some more and coax the data to be presentable in an understandable and scientific way. I want to publish the data along with ways in which someone can repeat it, verify it, scrutinize it, and critique it.
I can share some of it with you on a private basis, but I am not ready to do so openly until I'm ready. It answers this and several other things. But spoiler alert: the evidence shows the limo slows and that the Zapruder film appears to be consistent with a real, uncut film, at least in the portions I can study.
This invitation is open to anyone on the forum who wants to see the data rather than some guessing and supposition.
I am primarily researching an as-yet unknown feature of the Zapruder film, but one of the side benefits is that is can be used to calculate the limo motion pretty accurately ... or particularly, find anomalies in the motion of the limo. It could, conceivably, find anomalies in the Z-film itself, if there are any.
My research is somewhat like the work of Alvarez on the limo, but, unlike him, I show my work ... and, as other researchers have postulated, he was not completely accurate. If you look at his *HSCA report, Figure 7, you can tell without processing the data that he's cooking the numbers. (Watch his inaccurate use of the word "exactly" in the same sentence as the not-so-exact "few.") The epitome of absurdity in Alvarez's work on the limo stop is his Figure 7 caption: "Individual measurements are accurate to about +/- 4 inches." Yeah, ok, whatever. Meanwhile, the same graph has "~12 mph" and "~10 feet" which sounds pretty approximate! What hubris. He did not do anywhere near the research to come up with +/- 4 inch accuracy. That's ridiculous.
See A physicist examines the Kennedy assassination film (from LBL) as it is better than HSCA's copy.
https://imgur.com/a/rOXS4zd
I had a look at the Alvaraz graph showing the jfklimo braking at Z300. My own measurements of the limo's motion (frame by frame from Z270 to Z312)(relative to gawkers) duznt show any obvious hard braking at any frame. I firstly measured Davidson's frames, but i then realized that this was fraught (Davison's frames are altered), & i then measured my own file of (original) Zapruder frames (which as i said contradicts Alvaraz) (ie there was no hard braking).
Anyhow, braking at Z300 duznt do the trick. Hickey's say shot-5 was at say Z312. Shot-4 was at Z309.25. Shot-3 was at Z306.50. Shot-2 was at Z303.75. Shot-1 was at Z301.00. Based on 400 rpm, & 18.3 fps.
Hickey picked the AR15 up off the floor, muzzle facing say left, & Hickey then raized the AR15 up over O'Donnell's head, swinging it around to his right, while trying to stand up (he had been half sitting half standing hi up on 2 leather cases positioned on the back seat), while shuffling to his right to get clear of O'Donnell's seat, to enable Hickey to turn around to his right-rear & look for Oswald & to return fire. During this time Hickey accidentally squeezed the trigger, starting an auto-burst of say 4 or 5 shots. The squeeze was probably due to falling forward. The falling was probably due to Kinney braking, together with Elm St having a grade of more than 3 deg.
So, Kinney's braking (of Queen Mary) might have been say 0.22 sec before Z301.00, which is 4 frames, which is Z297.00.
And Greer's braking in the jfklimo might have been say 0.22 sec before Z297.00, which is Z293.00.
So, the jfklimo braking at Z300 duznt do the trick. But, if Hickey fired only 4 shots, then the Z293.00 becomes Z295.75.
And, if the AR15-601 fired at 800 rpm instead of my estimate of 400 rpm, then the Z295.75 becomes Z299.86, which is close to Alvaraz's Z300. Some modern AR15s can fire at 1200 rpm i think.
Anyhow, Kinney might have braked as a reaction to seeing Greer touching his brakes (ie brake lights).
Or, Hickey might have fallen forward due to some imbalance, not due to any braking.
Or, Hickey might have accidentally fired due to butterfingers, not due to any falling.
If your measurements show that the jfklimo braked then that would be of interest to me.