The Quad Shot is a button combo for Halo 2.
The Quad Shot's button combo is: R, R, X, Y, Y, R, R, X
It is a more difficult but more powerful version of the popular Double Shot, who's button combination is R, R, X.
Quickly press R, then as soon as you have your R released again, quickly squeeze it again and this time hold, quickly after the controller has registered the start of the holding press X in order to have the two shots burst out at the same time or rapidly following each other. Timing is crucial, there is about 1/10th of a second to do so and within that second pace of the shots is also determined. Then cancel the reloading by the Cancel Reload and do the Double Shot again. This can be said as cheating. This also happens to work for the Halo 2 Vista game.