"Initial reports suggested two people were firing at each other. However, sources said investigators now believe the Secret Service agent was the only shooter."
According to that NYP article someone was a few holes ahead of trump with an ak47, secret service shot at them and they fled, dropping the gun. The suspect was then arrested in a traffic stop a 50 minute drive away from the golf course. I'll believe it when I hear it from a credible source.
The pics I saw indicate it was actually an SKS that was bubba'd (crappy polymer stock) so the dude probably bought it out of the back of someone's trunk or at a gunshow.
Last report I saw was the suspect left an "AK47 type rifle" with a scope, at a vantage point 300-500 yards from Trump's location at the time.
Now granted journalists don't tend to know a damn thing about firearms, or ask good follow up questions so who knows, it could be a DMR type of rifle like an SVD that the uninformed would mistake for an AK.
But if it's just a standard 7.62 x 39 AK pattern rifle, scoped or not, that's ... very optimistic at that range.