I saw a brief excerpt of comments by a Security chief (I think) who said that as a former President, Trump had a limited security detail but if they were protecting a current President the whole perimeter would have been guarded.

This article has just appeared and will be updated:

Donald Trump's alleged would-be assassin appears in Florida court on gun charges

I don't think the guy had a chance to fire because the Security guys saw the rifle barrel and fired off some shots and then chased him in his car and caught him.