Jack,
I guess you haven't watched The Lost Bullet.
It not only shows how the three casings ended up where they did by ricocheting off the boxes around him, but also shows how he could have sat on one of the boxes he had placed "just so," and leaned out from it to peek out the window up Houston Street while waiting for the motorcade to arrive.
By holding his rifle or having it near by, standing up from that box would have put him in his first firing position without his even having to take a step, and for his second and third shots, all he would have had to do was kneel down from that standing position.
-- MWT 
What if the original premise for the test is wrong? The WC and the HSCA both question the number of shots.
The marks and lack of them on CE 543 also leaves the premise in doubt.
Warren Commission
The physical and other evidence examined by the Commission compels the conclusion that at least two shots were fired. As discussed previously, the nearly whole bullet discovered at Parkland Hospital and the two larger fragments found in the Presidential automobile, which were identified as coming from the assassination rifle, came from at least two separate bullets and possibly from three. The most convincing evidence relating to the number of shots was provided by the presence on the sixth floor of three spent cartridges which were demonstrated to have been fired by the same rifle that fired the bullets which caused the wounds.
It is possible that the assassin carried an empty shell in the rifle and fired only two shots, with the witnesses hearing multiple noises made by the same shot. Soon after the three
Page 111
empty cartridges were found, officials at the scene decided that three shots were fired, and that conclusion was widely circulated by the press. The eyewitness testimony may be subconsciously colored by the extensive publicity given the conclusion that three shots were fired. Nevertheless, the preponderance of the evidence, in particular the three spent cartridges, led the Commission to conclude that there were three shots fired
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HSCA
"The buildings around the Plaza caused strong reverberations
or echoes that followed the initial sound by from 0.5 to 1.5 sec.
While these reflections caused no confusion to our listeners
who were prepared and expected to hear them
they may well
inflated the number of shots reported by the suprised witnesses
during the assassination" HSCA Earwitness Analysis Report, pgs 135-137
"All observers rated the rifle shots as very, very loud and
they were unable to understand how they could have been described
as a firecracker or backfire." HSCA Earwitness Analysis Report, pg 148
"'While recognizing the substantial number
of people who reported shots originating from the knoll the committee
also believed the process of collecting witness testimony was such
that it would be unwise to place substantial reliance upon it. The
witnesses were interviewed over a substantial period of time some of
them several days even weeks after the assassination
By that time
numerous accounts of the number and direction of the shots had been
published. The committee believed that the witnesses memories and
testimony on the number, direction, and timing of the shots may have
been substantially influenced by the intervening publicity concern
ing the events of November 22 1963" HSCA Final Report- pg 87
