M16

 is a gun. Reloading alternates between bursts of three bullets and a grenade launcher.

Trivia

 * The name of the synergy is likely a reference to the distinctive guitar notes prefacing each verse in "Fortunate Son" by Creedence Clearwater Revival. Both the song and all three guns are heavily associated with the Vietnam War.
 * The “Underslung,” and “Hip Fire Only,” descriptions are references to the movie, “Predator,” where the main character, Dutch, (portrayed by Arnold Schwarzenegger) never looks down his M16’s (w/ grenade launcher) sights and always shoots from the hip.

Bugs

 * Immediately switching weapons after initiating a reload while holding the M16 results in the sounds for firing normally and firing grenades being swapped.