Being handcuffed to him...a difficult strain to be sure. The clip demonstrates the awkwardness of trying to shoot Oswald in the head as has been suggested.
Then why would they try then? Why didn't they do it when he wasn't handcuffed to a cop from any of the numerous times he was walked out in front of the public in the previous 48 hours?
Ruby could've easily aimed for his head here. He got within 6 inches of Oswald before anyone even noticed him there! Yet he clumsily shoots him in the stomach. Oswalds arm is across his side protecting it, covering a lot of his abdomen, its a side shot with L. C. Graves in the way too.
Yet people seem to believe Ruby knew he would kill him and the plotters, after all this effort, were happy with this massive massive risk.
No one seems to be able to answer this though, to 100% guarantee that hes dead - why not wait until he got to jail and kill him there?