That's an input jack, so you can listen to the theme from Red Dawn on your iPod while shooting.
Or, Jamie's Got A Gun, Pistol Packin' Mama, Happiness Is A Warm Gun, Praise The Lord And Pass The Ammunition, Saturday Night Special, Big Iron, Bang Bang (My Baby Shot Me Down), I Shot The Sheriff, Lawyers Guns And Money, Annie Get Your Gun, Shot In The Dark, Hit Me With Your Best Shot, Shooting Star...
Mark H.