Using round numbers, an AR15 fired round speed at 3000 feet per second going 150 yards would take .15 seconds to target, Very lucky indeed.
A lot of people think the assassination attempt is a conspiracy. No one knows for sure but the tidbits of information available make for great speculation. If true; the Secret Service knew about the gunman a couple or few minutes before the 1st shot was fired. A spokeswoman for the SS said they did not go up on the roof because it was sloped and a safety hazard. That's got to be the most lame excuse I've ever heard. My personal speculation is the K9 ate their security plan. It's one big puzzle.