500 rounds from a twin-fifty? Probably less time then it took me to read your post and answer it.
I'm not sure about the 500 rounds, but I read it on one of these threads. It strikes me that this equates to 30 seconds to a minute of supressing fire, not even intended to hit anything.
I was never in action, but I suspect that a two way firefight would be a bit tougher than this. An I would expect that the gunners wouldn't stop until all resistence stopped. This whole episode makes no sense from Kerry's side. At best I can only imagine his supporters are mixing multiple episodes in their memories. I can understand that, but an extended firefight would have left more evidence.