Chimpoutapocalypse
Back a few decades, for riots, there were riot guns - short barreled repeating shotguns with cylinder chokes.
Still seems like something to consider to me. These days, with shotshell technology being what it is, you can pretty much change your pattern by changing loads. The cheapest 00 buckshot, like S&B or Rio, has soft lead pellets that are unprotected in the shell and therefore deform easily upon firing. The pellets come out of the muzzle looking as if they have been faceted, and the flats make the pattern scatter faster ... if this is what you want.
The brand name ordinary buckshot from Federal, Fiocchi, Winchester, Remington etc. costs a little more but will usually yield better patterns. The pellets are made from harder lead alloys, or in some cases plated (Fiocchi uses nickel) to make them stand up to the stress of firing better. In addition, they have better wad columns, shot cups or shot collars, and sometimes buffering between the pellets to help protect them from the impact of firing and the abrasion of contact with the bore. That means the pellets are more likely to exit the muzzle in more spherical form and thus pattern better.
The 'premium' buckshot loads all have more of the above ... and that is reflected in price. The common attribute of new generation premium buckshot loads is a relatively new one piece wad design called FliteControl. This wad holds the pellets together for a bit after they leave the muzzle, as well as giving the pellets superior protection in the bore. The wad has fins which pull it away from the shot column shortly after leaving the bore, allowing the pellets to travel to the target in as tight a pattern as possible. The pellets themselves are of extra hard lead alloy, plated with copper or some other harder metal, and buffered in the shot cup by ground plastic known as 'grex.' For some these shells are counterproductive, as they produce patterns considered "too tight." IMHO that's not possible, but to each their own.
A good repeating shotgun is one of the least expensive "serious" long guns available. A new Chinese-made clone of the Remington 870 can be had for about $200, and numerous good used pump shotguns are available that will serve the purpose for that much or not much more. Semiautos are repeaters too, but generally semi's will cost at least twice as much as pumps.
The down side to shotguns is recoil and low magazine capacity. Recoil can be handled with proper gun fit (most shotgun stocks are too long for many shotgun shooters), good recoil pads and good stance and gun mount, which requires some good basic training. Low magazine capacity can be overcome by learning to load the gun quickly on the fly. Shotguns have declined in popularity among law enforcement and the military in recent years but seem to be staging something of a comeback. As always, nothing is perfect but any person can make the best of the choices available to them.
