Incompetence is a hallmark of most assassins who are amateurs. Pros know what they're doing—most of the time—but your run-of-the-mill wing nut with a grievance usually knows nothing whatever about ballistics or firearms.
Look at the two women who tried to kill Gerald Ford: Squeaky Fromme (right) and Sara Jane Moore. Squeaky managed to get a Model 1911 .45 from someone in return for (ugh) sexual favors, but the John didn't tell her she had to pull the slide back to fire the first shot: she dropped the hammer on an empty chamber.
Sara Jane (left) used a revolver—a S&W Model 29, according to accounts—and she even managed to get a shot off, but thanks to a heavy double action trigger and an intervention on the part of a bystander, she missed.
Then there is Mehmet Ali Agca, who tried to off Pope John II in 1981, the same year Reagan was shot. Agca was allegedly a trained killer from the Bulgarian Secret Police (I am not making this up) and certainly used an adequate caliber, 9mm Parabellum, which is endorsed by 9 out of 10 European professional assassins. But he used Bulgarian military issue ammunition with full metal jacket bullets. He perforated the Pope in several places and sent him to the hospital, but didn't kill him.
Sometimes, they do know what they're doing. The man who whacked the Swedish Prime Minister Olaf Palme in 1985 certainly did. He walked up behind Palme and shot him in the back with a .357 Magnum. To this day, there remain unaswered questions about this event, mainly why anyone would want to assassinate a Swedish politician in the first place.
Most successful assassins use rifles, not handguns. Lee Harvey Oswald (who, no matter what you think, acted alone) and James Earl Ray, slayer of Dr Matin Luther King, did.
Here is a point to ponder on this subject: John Hinckley was never convicted: he was found "innocent by reason of insanity," and to this day is theoretically still in custody at the Home For The Bewildered in Washington; but he recently won a court case allowing him unsupervised visits with his family on weekends. No word on whether he has called Jody Foster to ask her for a date, though. Sara Jane was released from prison at the end of 2007, a) contrite and b) aged 77 and presumably no longer a danger to President Ford, since he's dead.
Squeaky Fromme remains in prison. Though she has managed to escape twice, because her beloved Charlie Manson needed her, she was recaptured, and is now in prison in Texas. She is eligible for parole but refuses to apply for it. Of course, now that Charles Manson has gone to reside with the morning stars, she may change her mind.