Barrel length doesn't determine mechanical accuracy.
I think you'd get an argument out of the bechnrest crowd regarding that statement. It's not by accident that virtually all the barrels in the Light Gun and Heavy Gun winner's circle are precisely 21.75" in length. I don't pretend to understand it, but I do read Precision Shooting frequently.
Yeah, the benchrest crowd probably would have a lot to say about things that don't relate much to hunting arms, I suppose. BR shooters (far as I know - not being one myself) put in the effort to dial in things like matching powder burn rates to barrels and vice-versa. Most hunters won't go very far with that. I suspect (again, I don't know) that details like that may be a factor in determining optimal barrel length in their game. And most hunters won't spend the money it takes to make a rifle barrel truly accurate anyway.
But when it comes to guns appropriate for big-game hunting (which may or may not be the interest of the OP) and not looking for accuracy differences measured in hundredths of an inch, I don't see any comparison that shows where length will make a difference - on scoped barrels. I've seen too many "handrifles" that shoot just as accurately from a rest as hunting rifles that are often the subject of much boasting, for anyone to convince me there is any practical difference in mechanical accuracy.
