I carried (and used) a shotgun on the street for many years, until I finally retired out of police work. Any time we had a weapons call (or I anticipated the need for firearms) it was in my hands - a standard riot gun, four shot tube, bead sight - an utterly reliable Wingmaster (this was before Remington ever came out with a "Police" model..). In my later years I also had an old cloth bandolier (meant originally for the M-16..) with quite a few extra rounds (30 to 40) in case I had to move into a situation involving military grade weaponry on the other side of things... The extra rounds were only meant to allow my guys to clear the area if they were out-gunned (a real possibility back in the cocaine cowboy era - and the years that followed down here in paradise..). To put it mildly, most police forces were never meant to handle situations where they were badly out-gunned before the party started... Thank heavens I never needed those extra rounds...
I know that for most, their defensive (or assaulting) shotgun training and use will never be anywhere else but a range somewhere - maybe in competition - but mostly range time. Any time I hear about how quick a gun can be operated (and on this thread there's even someone talking about "double taps" as though they were using a handgun....) I just shake my head since a shotgun with standard 00 buck rounds is the closest thing to Thor's hammer that I know of - within its capabilities... For me a shotgun is a close quarters weapon -with the old poppers we used (a mix of Remington and Mossberg standard riot guns) with Improved Cylinder barrels was for ranges of less than 15 meters, generally. A single center of mass hit with what we carried was a genuine fight ender - no need for a second shot. What was needed was the ability to smoothly move from target to target (if more than one opponent was involved), then take the time to line it up before making that final shoot/don't shoot decision. Yeah speed had something to do with it - but I always counselled my guys to line them up first before taking the shot... and the only addition to that was to "aim just a bit low" since when you're scared to death and your heart feels like it's in your throat... most will shoot high if they don't compensate a bit...
Unlike most encounters by citizens in a self defense situation - cops on the street will find themselves pointing weapons at offenders on a regular basis, time after time - when no shot is fired (or would it be correct to fire...) -so I had to be very sure - and know that I was always perfectly safe - even while pointing a gun at another individual.... That meant I never, ever took the safely off of my loaded and cocked shotgun until that last moment, period. It served me well in all those years with never an accidental discharge until that one day when I did fire a single shot - and ended a young man's life... All these years later (almost forty now) I still wish I was somewhere else that day.
For those wanting to become competent defensive 'gunners - learn your weapon, make sure you pattern it with the ammo you're using so you know exactly what the spread of your pattern is at a given range (those old poppers we used had a pattern that dispersed at exactly one inch per meter from the muzzle - at seven meters you had a seven inch spread...), then spend most of your money on ammo and practice. Speed and maneuver is life itself on the street when weapons are involved.
By the way... I can't remember how many times I pointed a handgun at someone on the street (and almost half that time was pretty much ignored...). I do clearly remember that any time I pointed a shotgun at someone in close quarters I was almost never ignored with only one or two exceptions (and I killed one of them...).