A coupla things....
Back in the late 70s, I had some contacts at Aberdeen Proving Grounds. The Army had some brainy folks playing around with munitions and making lots of loud noises.
One project was an ambush buster. They tried various ideas and combos. One approach had two full auto shotguns with big drum mags mounted together in tandem with a full auto 9mm sub gun in between loaded with only tracers to direct fire.
The 9mm had a slow rate of fire(for full auto) and the shotguns were like 90 RPM. Not fast by military standards, but these folks spoke in awed tones of just how much damage a 5 second engagement resulted in. Bbls were regulated to cover an area totally at maybeso 40 meters.IIRC, we're talking about one 00 pellet per square foot. BTW, this was and is declassed.
They had another ambush buster. It was a small turret with a series of short shotgun bbls mounted to it, looking something like an old time naval mine. The bbls stuck out at angles maybe 35 degrees above and below horizontal and were good for one shot w/ no reloading mechanism. A simple electric control had all the bbls going off simultaneously turning the area adjacent to the vehicle into a very dangerous place. I understand the South Africans had something similiar during the Apartheid period. With People's Armies and tribal warfare burgeoning, this could give a civilian vehicle a chance against a squad or two of banditos or Freedom Fighters, depending on your viewpoint. Kind of a super Claymore....
A battered Winchester 97 was part of our crew equipment in the late unpleasantness in SEA. It was used on guard duty and as a backup. A MM79 was the other part of that backup. I stuck with my highly irregularly acquired M-14.
Shotguns seem to appear like magic in combat zones. I saw plenty of pumps, some A-5s and even some doubles worked into "Whippet" guns.I doubt many of same were regarded by their bearers as first line weapons.
The Phillipinos during WWII turned shotgun shells, nails and irrigation pipe into tools to obtain a nice shiny new Japanese Army Rifle. This was called a Palintid, and after the war ended, many variants were developed, including full auto 410s.
Going back a bit further, plenty of Confederate cavalry like Jubal Early's hard riding men used M/L shotguns, oft shortened, to even the odds. One was on display at the Gettysburg museum had bbls about 14-16" long.Imagine a company of Reb Cavalry coming at you, firing two volleys and then going to saber and sidearm. A common load was 60 gr of FFF or FF, and 5 pistol balls(Probably Navy sized, say .375) per bbl.