From 300-yards distance, the degree of angle variance in the shot's trajectory to DJT's death is about 1000 th of a degree. IOW, if the shooter had fired the shot just 1000th of a degree of angle differently, the shot would have hit roughly dead-center of DJT's head. 

The shooter was either very lucky or very good. Either very good training/skill in shooter evading SS long enough to get shots off, or lax coverage for the event. 

The latter is probably the most troubling- in case he was a hired hand (by enemies). Since there is no I.D. on the shooter, foreign actors can't be excluded as possibilities. 

A strong motive for a political assassin would be to either end him, or disrupt/end big Trump political rallies. What are the odds the big rallies will end? heh