20mm leaves the muzzle at 2,350 mph. F-16 on full afterburner is doing a little over 1500 mph. The F-16 can maintain that speed, the 20mm round can't.
It's long been known that you should not fire the guns then stay in a straight line UNDER the bullet path. European allies had the same problem back in the 60s. There have been several reports of US pilots doing the same thing. The other issue that pilots have had is when doing a 10 degree strafing run, bullet ricochets have been know to happen and the pilot has shot himself. I have heard that US protocol is no longer to do 10 degree strafing runs (at least during practice).