The top speed for the F16 is roughly 1500 MPH. That's 25 miles a minute. 300 miles is 12 minutes. If we figure that they took off at 9:56 they would have arrived at 10:08. Since the flight crashed at 10:03, I doubt they could have gotten cannons on at 125 miles.
Even at 9:49, and allowing 5 minutes to load the ammo, you are still looking at an arrival time of 10:06. I've no actual knowledge of how long it takes to get ammo out and loaded, but 5 minutes would be blazing fast. Even if just bullets. You'd want at least a thousand rounds and those things are heavy. I remember having to pack 50 cal rounds, when I had to, and damn those were heavy. Sure, I did dump them on the A-gunner, and anyone who wanted to shoot, had to pack their own can, but it's not like something you can grab and sprint across the tarmac with.
Basic math makes the theory impossible. But that doesn't stop the theory, does it?