Not even a specialist weapon - it would need to be a completely custom job, from the rifle to the ammo.
You mean a custom installation. Check Youtube. You will find a small army of people who have installed very effective suppressors made from flashlights, oil filters, beer cans and other equally exotic items. Watch this video of a guy using an oil filter. Move the timer to 1:20
https://www.youtube.com/watch?v=haiqFcIXTqs
Making an effective suppressor is not that difficult.
I am unaware of any subsonic 6.5mm rifle ammunition (and you are limited to this calibre as this is ONLY size of bullet that was recovered) that is or was available, so this would need to be manufactured, in addition to a rifle to fire said ammo.
That is incorrect. We have no idea what the caliber was of the apparent missed shot circa 150-160. Nor do we know the caliber of the shot that caused James Tague's minor wound and smear on the Main St. curbing. And CE399 could not possibly have been the bullet that wounded Connally and probably JFK. That fact was confirmed by Connally himself, the Dallas DA Henry Wade and the officer who was guarding Connally's room. It was further confirmed by half a dozen others. This article explains in detail.
PLEASE read it in it's entirety. The evidence in this article is extremely important.
http://jfkhistory.com/bell/bellarticle/BellArticle.html
It is true that fragments found in the limo were alleged to be a match with Oswald's rifle, which may be because he actually fired that shot. But it's important to remember that to accept that, one must have a certain amount of faith in the FBI, that they presented the actual fragments, which might be a bit hard to muster after reading that article.
Mr. Harris' scenario of an another shooter requires:
a. Close co-ordination between the shooters (and with the lack of comms equipment you are down to a previously agreed time and wristwatches);
No sir. This was not the Green Berets. This was a few thugs whose plan was to start firing when the target came in range. No communication or coordination was necessary. Do you think the Mafia required walky-talkies on the their other hits?
b. One shooter to leave physical evidence, while the other leaves none;
Yes. Oswald was not recruited for his shooting skills. He was recruited for his communist/Cuban legend. If the crime could be blamed on a commie who was a leader in the FPCC, perhaps the US would declare war on Cuba and get rid of Castro, freeing the mob to re-establish their lucrative casino business. So, it was important that Oswald or someone, made sure he left a large evidence trail.
And the plan almost worked. The DPD in fact, stated in their indictment of Oswald that he was part of an international communist conspiracy.
c. The second shooter to have a custom rifle with a custom calibre, and a suppressor that can be used to make a longer and more challenging shot that the other shooter.
No sir, the last suppressed shot that I can see, was at 223, when the limo was about 100 feet from the Daltex. After that, the high powered rifles took over and were extremely loud. Of course, they were not suppressed.
It is easy to see the difference in the reactions to the early shots and the ones following 285 and 313. Of course, people reacted to one of the early shots. Their reactions were spread out over roughly 100 frames, from 169 when Jackie began to react, to app. 270 when Kellerman began to turn to the rear.
The reactions following 285, all began in the same 1/6th of a second and at exactly the same instant where Alvarez identified Zapruder's reaction. These are the reactions:
http://jfkhistory.com/ducking3.gif
and this demonstrates that the reactions all began in the range of 290-292, or 3/18ths of one second.
http://jfkhistory.com/simultaneous.gif