"Demonstrating" the shotgun, and then asking someone to "disburse," would be the felonious Aggravated Robbery - Deadly Weapon in Texas, and a similar offense just about everywhere else. Sorry to be seen as a grammar nazi, and usually I do not correct others' grammar, but seriously, this is not useful, as written. Words indicate intent; better get it right, whether spoken or written, and let's not forget that rioters will video-record their activities, preserving forever a mis-spoken word.
Assuming the protagonist has, instead, demonstrated the shotgun, and requested that the antagonists disperse, well, at least we are not requesting the antagonists' money, but unless the antagonists are already on private property, controlled by the protagonist, the protagonist is likely to be in legal hot water.
OK, now, assuming I am protecting private property, or territory I am legally sworn to defend, against unarmored, lightly-clothed antagonists, who are a true threat, either 00 or #4 buck should suffice at 10 to 20 yards. Beyond 20 yards, I would prefer the larger Buck, or perhaps 000 Buck.
In actual practice, I keep 00 Buck in my 870P duty/defensive shotgun. My chief authorizes 00 and #4, and slugs. At times, I will have slugs and #4 with me.