If you hit Donald Trump with any sniper rifle calibre from 150 m distance ANYWHERE on his body, the man's toast. He's a 80 year old jellybean, held together by his personal BigMac/Adrenochrome mix.
But the dude likely played too much COD or CS and simply got convinced that Trump has too much HP and you need to go for the headshot. Motherfucking 
This definitely isn't something that was planned in advance. Some spur of the moment shit. Dude went hunting with an iron sighted AR-15.
i think americans are just bad at spectacular acts of violence now. we watch a lot of tv. we expect it to be easier than it is. remember that guy that attacked an FBI building with a nailgun, thinking the nails would penetrate bulletproof glass? or the nazi dipshit who wore a ski mask wrong and didn't shoot anyone?
If it were planned, he'd have the right equipment for the job (perhaps a suppressed .308 hunting rifle), have it range zeroed for expected distance and know where the SS shooters were setup.
Why suppress tho? You gotta figure you get one maaaaaaybe 2 shots if at all before you're blown away
"I'm just gonna shoot at the presidential candidate and see what happens" feels really wrong. He missed his head by a few centimetres. How is all of this even possible.