I've been intrigued by the idea that Oswald was trying to achieve two things with the first shot; Checking the scope, which he probably didn't trust, by aiming at a fixed object (possibly the manhole cover); and standing well back from the window.
By standing well back from the window the sound would be less obvious to people on the street, and would also create a sense of doubt or confusion when the next two shots came, as proved the case.
The lady "manning" the TSBD switchboard on the second floor testified that the first shot "sounded like a cannon and shook the west side of the building".
All speculation of course, but it could explain why many people missed the sound of the first shot.
There was a guy on McAdams' website who used to argue that Oswald's first shot was a zeroing shot but that never made sense to me because it is impractical.
For starters, Oswuld would have to be able to see where the shot struck to know what kind of adjustment was needed to the scope. How could he do that with the recoil of the rifle moving the scope off the intended target. Even if he could see that, he would then have to take the time to adjust the scope. By the time he did that, the target would be long gone. Normally, when zeroing a rifle, you would do that from a bench rest to eliminate human error from the equation. You are also probably going to do the process multiple times.
It would have been nice for Oswald to have a perfectly zeroed rifle, but hardly necessary. He wasn't in a shooting competition. He just needed to be able to shoot a man at ranges of 88 yards and less. He seems to have done alright.
There are a number of YouTube videos that show how to zero a scope, and they all use a 3 shot process and a bench rest. Oswald only had 4 rounds and a limited time to fire them.
