Once upon a time it was necessary to have some amount of choke in even a short (18 - 21") barrel to get good patterns at longer range with the buckshot loads that were then available. Since most riot or security type shotguns had CYL bore barrels, installing choke tubes was the way many people chose to add choke to short barrels, though I have seem a couple that had PolyChokes installed before choke tubes became widely used.
Remington started making factory short barrels with fixed chokes several years ago. Now many of Remington's Police or smoothbore slug barrels with bead or rifle sights have fixed ImpCyl chokes. And Remington has made Police shotgun barrels with Mod or even Full fixed chokes. In fact I just missed getting a short bead sighted Remington full choke Police barrel from CDNN a couple of weeks ago (they sold out), and I have two 18.5" tritium rifle sight Police barrels with fixed Mod chokes.
Now, much of the time, all you have to do is choose your buckshot loads to adjust your pattern, if your barrel is a CYL or ImpCyl bore. Want tight patterns? Try a load with FliteControl wads. The 'standard' loads from any big-name manufacturer will usually give middle-sized patterns, and the cheap, soft lead pellet, unbuffered, unprotected by shot cup or sleeve buckshot loads (S&B, Rio Royal etc) will usually give wide open true scattergun patterns.