I'm not a fan of shotguns for anything but bird and clay pigeon shooting.
In LE I had a Remington 870, 18 in cylinder bore "riot" gun I carried in the car, but seldom used it. Seemed it was always in the way.
Part of my LE Bomb Squad duties was making breaching charges for our SWAT (we called them SRT) guys. Breaching charges for cutting openings in walls and opening doors.
Slugs are very poor for taking off hinges. If not hit dead on, they'll often glance off the henge. Also they over penetrated. You don't know what's inside the room until you get in. You don't want a slug flying around inside.
I used a fine sawdust mixed with wax to cast slugs for breaching. They'd take off the hinges but totally come apart where if anything penetrated, it was only dust from the slug.
I do use slugs and buckshot now, but only when required in Three Gun Matches. Nothing fancy. I use the Lee 1 oz slug. I can keep them in the head area of the IDPA type targets at 30 yards out of my smooth bore 18 in 870, so I can do in pepper poppers at the ranges fired in such matches.
The same with buck shot. Nothing fancy, I use a Lee round ball mold, .319 in diameter that cast 48 gr. balls. I stick 9 of the little suckers in my AA Hulls and I have my buck shot for 3 gun shooting. I can keep all 9 pellets on the IDPA targets at 25 yards.
But, past that, or past the distance I can shoot slugs, I'm better with the pistol/revolver. Or it too far for the handguns, I'll just pick up a rifle.
I understand that some places require the use of shotguns for hunting. It that case I'd probably work up some rifled slugs or get a rifled barrel for my shotgun.
As to penetration, some talked about busting through light armor (car doors, engine blocks etc)...........slugs are too soft and big. In WWI when those light armored tanks first started showing up, GIs found they could penetrate the armor by pulling the 30 cal bullets, and re-inserting them in the case backwards, they out penetrated even the armor piercing rounds.
Shot guns are fun to play with, but I don't think I would want to stake my life on one.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071