Just because someone says they were the best doesn't mean it's true.
Using a scoped, bolt action rifle, I once shot 3 times at a deer that was running away from me through a timber cut field. One hit by itself was fatal requiring no tracking, one hit by itself would have meant tracking it for some distance but would eventually have been fatal. One hit by itself was a minor flesh wound he would have quickly recovered from.
Unlike that buck bouncing through brush, the target profile presented by JFK was essentially stationary.
From the book depository Kennedy was a moving target. They actually had marksmen try and hit a Kennedy dummy in the 8.4 seconds that it took to get three shots off from that angle. Can’t be done.
You might be the best marksman in the world if you think you can pull it off.