FYI: If you haven't read the book Tom Givens recommended a little while back about the Dallas PD Shotgun/Stakeout Squads,
Holloway's Raiders there are a remarkable number of incidents where an officer jacking a round of buckshot into the gun resulted in the surrender of a badguy.
There are also a remarkable number of situations where officers missed with shotguns (perhaps not remarkable to folks who have shot a lot of shotguns, but remarkable for those that think you "can't miss").
And a number of incidents where even when officers had the drop on a bandit, they either missed their shots, or didn't take the shots.
But as it were, Holloway was not an advocate for leaving the gun on an empty chamber, nor calling out verbal commands, etc. The infamous quote by Holloway in the book is, "I don't believe in shooting a hi-jacker without warning, but I think the click an 870 coming off safe is warning enough."