Assuming the incident you reported was a valid one, I'm disappointed in the results of your reporting a serious safety violation.
 Even IF they perpetrators were gone when the Rangers arrived, you can't fire that many rounds without missing some fired brass, or the results of your target practice not being evident somewhere around the campsite.
 Brass would tell caliber, possibly make of weapon and unless they wore gloves, finger prints could be obtained.
 From you conversation, I think they just doubted the whole incident when you said you heard Automatic weapon fire? Too a person unfamiliar with firearms, a semi-auto ( you squeeze the trigger once for every shot fired) can sound like a Full Auto, if the person firing is proficient enough. They also make crank handle adapters that allow you too fire a semi-auto so fast, that they sound like full auto.
 However, if anyone has served in the military, or been exposed too gunfire from a nearby firing range, the sound of a semi-auto and full auto are much different once you've heard both a few times.
 I'm wondering if there wasn't something going on that for whatever reason the authorities wanted too keep under wraps?
 I'd like too see that campsite, there must be plenty of evidence waiting for the forensic guys IF any ever show up ?
I hope it doesn't take a fatality before its investigated and the facts made known.