everything I've ever read about slug barrels, from shotguns....12ga mainly is that you only need 21"

I assume 12 or 20 ga the barrel is so large in diameter that it quickly gains its speed, and burns all that powder well before 26 or 28"

I was curious about a 24" vs 21" vs 26" and if anything over 21 or 24" there is a strong liklihood that a longer barrel will only slow your slug down, may increase accuracy, but velocity no, I wouldn't go over 26", and probably 24" if available.