Delayed Reload is a button combo present in Halo 2. It is executed by pressing R + X, off-host. At the beginning of the magazine of your rifle, hold fire and press X for the first shot, then you can fire a few rounds before it auto-reloads. If you learn the timing and press X every first shot, you can keep going without reloading. With the Shotgun, you can do this all the time; after fired rounds, it will reload until it's full, and then you can fire whenever you want.