Years ago, the NRA Dope Bag did a test on ballistics vs. barrel length on shotguns.
They acquired a Marlin Goose Gun with the 36" barrel, and attached a threaded-on choke.
They patterned and chronoed the gun, then cut one inch off, reattached the choke and repeated the tests. They continued until the barrel was around 12 inches long.
The result was, the NRA said that anything ballisticly that was going to happen in a shotgun barrel, would happen within 18 inches. Anything over 28 inches, and you started to LOOSE due to friction.
The barrel was around 14 inches before everything really started to fall apart.
The advantage to a long barrel was in "point-ability".
The idea that longer barreled shotguns shoot farther or "harder", is a left over from blackpowder days, when longer barrels burned the powder more efficiently.