Perimeters, Stalkers and Stabbings During Commercial Breaks
I have worked security for athletes and celebs. Worked the Olympics. Dealt with stalkers. The crazier the attacker the more unpredictable their tactics are but rushing in is common. Unlike snipers and others with firearms training, stalkers tend to want to get as close as they can to their target, even if the are using a gun instead of a knife. For the obsessed, the attack is an intimate act. They want to be known for their crime and thus merge their identity with the target or the act itself. Syphilis and 'delusions of grandeur' are very common in stalkers.
March 30, 1981 at that same Hilton Hotel in Washington DC, Ronald Reagan and three others were shot by John Hinckley Jr. Although obsessed with Jodie Foster for months, Hinckley's planning was minimal. He bought a cheap revolver ( a .22 caliber Rohm, aka Saturday Night Special). Pushed his way toward the front of the rope line and emptied the gun, permanently disabling James Brady. It was the end of rope lines and completely changed the way Reagan interacted with the public.
Hinckley stalked and was going to shoot Jimmy Carter in Dayton Ohio. Got within 20-feet. Chapman was similar. No perimeter. Pushed in close. Emptied the gun, a .38 revolver. Both were celebrity-crazed but one killed his idol while the other shot a high profile figure in an attempt to form a relationship with an actress. That their targets and motives make little sense makes it even harder to defend against such attacks.
When I worked security at the Olympics they told us it was "highly likely" that multiple security events would happen. The chances were high in part because of the sheer number of potential targets combined with the international spotlight. Any scheduled appearance of a targeted person is a magnet for stalkers and attackers and the Olympics is literally hundreds of events scheduled a year in advance. They gave us guidelines; one of which was 'no one gets in without a ticket' and anyone who does needs to be arrested.
I was working one of the gates at Women's Tennis. Father hands me four tickets. I let four people through the turnstile. Fifth person says "That guy pushed into the middle of our group. He's not with us." I let that guy in. Locked my gate and ran after the interloper. He has crazy eyes, the ones where you can see white under the iris. He is not stopping. Not talking to me. I yell at an usher to "Get LAPD now!"
I'm trying to stay in front of this guy. Watching his both hands and I have my left arm up over my head so the LAPD can find us in the crowd. Finally he speaks to me in German. Says he is going to use the badezimmer. I answer him in my crude German: Nein. Sie haben keine ticket. Sie mussen ausgang!" and that makes him even angrier. Now he is bumping me. His energy is totally hostile and kind of panicked. I'm watching his hands. Trying to remember my training, eg if knife attack grab their thumb. LAPD comes and takes him in cuffs. I get word later in the day that he is one of the known stalkers of Steffi Graff. No word on whether he had a weapon.
Years go by. April 30, 1993. Monika Seles gets stabbed between the shoulders during a commercial break at a tournament in Hamburg Germany. They show the attacker -- the hair on my arms literally stands on end. Same guy. Asked why he did it, he famously answered "I stab for Steffi."
Worked many other incidents at many other venues. I am not surprised that some attackers get in. Don't want to list the common vulnerabilities at most venues but in general, places where attacks are not expected, (a tennis tournament, apartment entrance) are especially vulnerable. Most celebs have multiple stalkers and they are mixed in with obsessive fans. Politicians attract haters and hecklers and protest and serious attackers can be mixed into those groups.
Seeing the attacker last night running through the first perimeter make me think this is not a fake incident. As crazy as attackers can be, this one fits into a fairly common pattern -- waiting as long as they can for an opportunity and then rushing. Obsession + minimal planning + no prior history of violence. That he had a shotgun tells me that he intended to get as close as he could and that he didn't have or trust his marksmanship. Nor did he care much about who else was going to get shot by him.
= new reply since forum marked as read
If this person acted after being encouraged by undercover agents to do so is an obvious example. Take someone already unstable, and use professionals to manipulate him into going through with it. I mean, the FBI is sorta famous for this sort of entrapment thru encouragement of illegal acts.
Not saying that's LIKELY, mind you, but it could be both "real" in one sense, but "fake" in another (the POTUS being there in the first place is, as we all know, unusual. Trump had never been to one as POTUS, and it's not been common for sitting POTUS's to attend).
And the "Trump was the target" storyline we're all being fed could be "fake" as well. Even if the dude's actions were entirely unknown in advance by any authorities and there was no conspiracy, that storyline could (pretty easily imho) be a false one.
Funny though, isn't it, the way that when someone on the right is targeted by someone violent, it's always that THIS person was in full control of their faculties, acting out because of "zomg overheated rhetoric by Teh Left!!11!" but when it's roomful of school kids or a church full of minorities, it's always a case of a "mentally unstable individual" 
Sorry if that was unclear 