Having taken a good look at various types of buckshot loads fired from both a short riot gun with no choke, and a full length shotgun with a full choke, We appeared to have a good handle on how buckshot is going to behave at the target.
But.... one test was left out, even if improbable. What happens when buckshot is fired through a rifled shotgun barrel meant to spin slugs? Will we see the 'Donut of Doom' so commonly espoused? Will it effect the pattern at all?
A rifled shotgun barrel is designed as a slug thrower, not a 'shot' thrower. Sure, it will go bang with shot and stuff will come out the end of the barrel, but what happens after that is not so clear. The spiral rifling meant to impart spin on a solid slug and stabilize it. It will impart the same spin on the shot charge. It must have an effect, but exactly how much of one was unclear to me till today.
Using a single shot H+R rifled slug gun in 12 gauge, I fired three rounds of #4 buck at a silhouette target. The #4 buck has 24 pellets per shot charge, and leaves sufficient 'pattern' for analysis. Three rounds.... nearly 75 pellets, and enough to make a judgment on what happens (click on the target photo to enlarge).
The firing distance was about 35 feet, and roughly 30% of the pellets hit the silhouette with any effect. The rest.... decorated the hillside. The conclusion is clear; a rifled barrel will force the shot to spread at a very rapid pace. Where a cylinder bore shotgun will place all it's buckshot on center mass at that distance, a rifled barrel will splatter most of the pellets far and wide, with only some hitting the target in a useful manner. The 'donut' of shot did not appear on this target, and the pattern was without... well.... 'pattern'.
Would I use it for defense? If there was no other choice, yes. Given any choice at all I would prefer a smooth bore of any type over rifled for shooting buckshot.