I'm not sure how I overlooked this. I have enough experience with AR-15s (hence the name) to know that this doesn't make sense.
Only two explanations:
* The shooting was staged. (Unlikely)
* There were multiple shooters. (Likely)
Stay safe.
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Everybody watch out. OP is a navy seal with 420 confirmed kills in ww2, Korea, Vietnam, and Iraq. He knows how ar15s work therefor all your arguments are invalid.