No matter how good a shot, shooting to disable is very difficult. It's like the old Westerns where a cowboy shot the gun out of a villains hand.
Even if you are great at shooting, there are so many factors making disabling shots very unlikely: Wind, temperature, muscle micromovements, even the rotation of Earth can factor in... a disabling shot could easily become a fatal one.
The Secret Service sniper was able to quickly kill the perp, after the perp fired his rifle. People are saying that the perp was not skilled at shooting President Trump. I disagree.
From various reports, it appears that after witnesses said there was a man climbing on the roof with a rifle, a cop did try climbing the ladder and yelled at the perp, asking what he was doing. The perp spun around and pointed at the cop, and the cop lost his footing and fell back from the roof/ladder. The perp then quickly spun around to target President Trump and fired several shots that missed killing him, only because President Trump had turned his head sideways. The perp shot without perfectly aiming because he realized the gig was up due to the cop being there.
The 20-year-old perp had managed to spin around and quickly get off shots that would have killed President Trump, if Trump had not changed his head position.