My understanding is that any venue where the president is located is considered a federal site and thus firearms are not allowed in his immediate vicinity. I don't know how far his immediate vicinity extends. This could all be nonsense, I am just going off this article.
SNIP "U.S. Secret Service spokesman Ed Donovan acknowledged the incidents in New Hampshire and Arizona, but said he was not aware of any other recent events where protesters attended with open weapons. He said there was no indication that anyone had organized the incidents.
Asked whether the individuals carrying weapons jeopardized the safety of the president, Donovan said, "Of course not."
The individuals would never have gotten in close proximity to the president, regardless of any state laws on openly carrying weapons, he said. A venue is considered a federal site when the Secret Service is protecting the president and weapons are not allowed on a federal site, he added."
Link to full story.