It wouldn't have taken much, just a slightly better shot.
On Monday, March 30, 1981 just sixty nine days into the Regan presidency John Hinckley Jr. shot then president Ronald Regan. In OTL he punctured a lung.
Suppose though Hinckley's shot went through the heart of President Reagan? Killing him almost instantly.
How would the world (as we've known it to be) have changed as a result of this?