I have zero experience with AR's and am considering buying my first one. Can someone explain the difference (in layman's terms) between piston vs direct impingement? I am not a tinkerer or a gunsmith and would probably leave whatever I buy in stock condition. I am hearing good things about Ruger's new AR, the direct impingement model, and I am wondering if this would be a good, reliable shooter out of the box for an AR newbie.