Tighten Screws

I took another friend from church ahooting today.  (He is slso thrilled with Home Gun Club and going to join.)  The AR-15 was reliably hitting the 200 yard plate last time.   This time always missing.   Why?  As I became discouraged and starting packing up, I removed the scope mount and discovered the screw was very loose.  The mount was quite freely up and down.  You do not need to be a shooter to immediately recognize why this would affect accuracy.