I wish i had listened to Mr. Leuba or any of the other quality instructors out their when I first got into training.
My First AR was a gas piston 6.8 spc. Why? Because I knew nothing and everyone who gave me gun advice told me that DI guns suck because they shit where they eat and 5.56 was only powerful enough to kill coyotes and poodles.
A few years later I realized that the people that gave me advice didn't even own any goddamn "assault rifles" let alone AR-15's! I also opened my eyes and realized that most if not all of the quality instructors shoot DI guns from only a handful of brands (BCM, Larue, DD, Colt, KAC, Noveske, and LMT) and shot glocks or m&p's. They use so many different types of support gear, vests, holsters, clothing slings. Yet their weapons are hardly diverse when you look at the entire group.
As vickers would say "id call that a clue."
It's hard to know whats good advice when you are starting out but instead of listening to their big ego buddies, new shooters should seek out a variety of top instructors and see if their are common themes. I know very few (one) instructor that primarily runs a gas piston gun. They are shooting every week and see millions of rounds go down range in their class….

