A warning just tells an intruder where you are so he can shoot you.
People keep repeating that, but is it really true? Maybe if he's in the same room with you. But consider this scenario:
BG is tippy-toeing around living room. I'm in bedroom. I rack the slide on the 870, and bark out "I AM ARMED AND WILL DEFEND MYSELF. POLICE ARE ON THE WAY. YOU HAVE THREE SECONDS BEFORE I START SHOOTING." I immediately move to the other side of the room.
BG doesn't know exactly which room the sounds came from, and even if he guesses correctly, AND is dumb or high enough to come on against the shotty, AND thinks he knows where in the room I was standing, I've already moved.
Am I going to jump out of the bedroom door and take a chance he doesn't have his Gansta Nine pointed in my direction? Hell, no.
Are the chances pretty damned good that he'll leave pronto and not chance it again? Hell, yes.
And even if he doesn't, he's going to have to come down the hallway and into the bedroom door, backlit by the light from the other end, and face the 12-ga. with the Surefire on it. See ya.
Oh, and if for some reason I can't get to the 870, the D-E .50AE is in the nightstand, and the only sound it will need to make to get into play is the muffled snick of the safety.