“He had at best a limited ability as a rifleman and no clear record of recent practice at even stationary targets.”
Well, he was a marine, and there actually is testimony that he was practicing at a firing range less than a week before the assassination. Plus there was the General Walker shooting, which I suppose counts as practice as well.
Showed what one motivated Marine and his rifle can do.
It has also been suggested that Oswald practiced his marksmanship in a nearby riverbed, which was apparently often done at the time. The problem though is that shooting at a moving target is a much harder proposition and was beyond Oswald’s training.