Or just say NUTS and mount a reflex sight.

From a purely engineering standpoint, with a one piece barrel the shorter barrel will actually be more accurate than a longer barrel. When a barrel is attached to the frame at just one end it becomes in effect a Cantelever Beam. Since a bullet transitting the barrel will cause that beam to bounce like a diving board, a shorter barrel will be "stiffer" in it's response and the end will deflect less during the transit event. I suspect that if a 2 1/2 inch 686 and a 6 inch 686 were both equipped with a good handgun scope and fired from a rest by a real good shooter, the short barrel would just trounce the longer barreled gun in group size. However, in order to prove this out completely, each gun would have to use a load that was optimized for the barrel length. Because Harmonics can have a distinct effect on any vibrating system and barrels do vibrate in response to a bullet transitting them. Ideally you want a load and bullet mass where the end of the barrel is passing through it's Neutral position when the bullet exits the muzzle. Ask anyone who's worked up loads for long range rifle shooting, just a 100 fps difference in velocity can make a distinct difference in group size downrange.

Now, if you are still young enough that you don't need to cheat with Optics, Sight radius Rules. Even the eagle eyed pro who never even smells caffiene will shoot more accurately with a longer sight radius. However, since most of us don't fall into the eagle eyed, caffiene free, category, I don't think the difference between a 3 or 4 inch barrel will be very noticable. The plain truth is that most of us can't come close to shooting to the accuracy our guns are capable of. Fact is for offhand shooting almost all of my practice is spent at 15 yards or less, for anything longer I look for a rest because most days I'm just not as steady as I would like to be.