First of all, IMHO the SHOOTER is going to have more to do with buckshot/slug performance out of a shotgun than the barrel will, since if the shooter can't put the load on target, it won't matter how well the barrel performs.
Secondly, the length of the barrel is basically irrelevant to physical performance in ejecting a shotgun shell payload at full velocity as long as there's enough barrel to burn the powder charge. With modern smokeless powders, that generally happens in about 14" of barrel, sometimes more, sometimes less. Barrel length on a shotgun has more to do with sight radius or ability to maintain a swing smoothly than any other factor.
Thirdly, the old rule of thumb about shotgun patterns at a given size per foot/yard of pellet travel is as dead nowadays as tailfins on a Cadillac. I'm not about to try and tell anyone what they are supposed to want from their shotgun/load, anyone should go after whatever performance they want, however they want to get it. I can tell anyone who has a reasonably open mind how to go about getting whatever buckshot pattern they want within the limits of current technology - it's simple. You want wide open patterns, shoot cheap unbuffered buckshot with unprotected dead soft lead pellets, like S&B or Rio, through an open choked (CYL) barrel. You want medium sized patterns, try the big name production buckshot with harder alloy lead pellets, shot cups and/or plastic buffering, and experiment with a touch of choke. You want super tight patterns, go back to an open choked barrel and shoot modern buckshot loads with FliteControl or equivalent shot cup type wads, hard alloy plated lead pellets, buffering etc. In short, I think people should be able to get what they want from their shotgun/load combination, and it isn't my responsibility to tell anyone what they should want.
My favorite 18" factory 870 CYL barrel will put 9 00 pellets from a round of Federal LE-127 00 into a four-inch pattern at 25 yards (75 feet) with boring repeatability. Closer in, it's a ragged hole. Personally I think it's my responsibility to hit what I shoot at, and not my shotgun's job to scatter buckshot all over half an acre. Most of the time, that is - if the time ever comes when the situation is "to whom it may concern" at night with no 'friendlies' out there to worry about, there's a case or three of S&B in the ammo pile for just such a situation.
I like Brenneke KO slugs (the non-sabot version) a lot, and have used them for several years now. Most any gun I've shot them out of will group them into 4" or less at 100 yards. They are hard lead, sharp shouldered slugs and cut neat round cookie cutter holes, and more importantly to me they will penetrate and not flatten out and stop like soft lead Foster type slugs. On the other hand, if you want a slug that flattens out and stops, shoot traditional Foster type slugs. Your call.
But as has been said repeatedly, the only way you can know what any given barrel will do with any given load is to burn powder and make holes in paper. There are some limits you can expect to see going in, and I think those have been covered here, but the ground truth can only come from your shotgun and the loads you want to experiment with on paper at the range.
Good luck and Stay Safe,
lpl
ETA - Brent, Brenneke is now making KO slugs in 20 gauge too. You ought to try some on the bigger hogs you have to deal with.
