Ok I am intrigued. Where might I find the other barrel configurations for this revolver? And are they easily swapped?What are the reasons for each length? I assume the longer the barrel the more accurate the sights and bullet travel?
I am uncertain what you mean by "find". But this should help:
This is what S&W offers now:
Revolvers | Smith & Wesson
©2021 Smith & Wesson
Just clicking on the Model 686 Plus shows you all of the available barrel lengths:
Model 686 PLUS | Smith & Wesson
©2021 Smith & Wesson
2.5", 3", 4.125" (most folks just call it 4"), 5", 6", and 7".
In a 686+ I have these:
2.5"
3"
4"
And for a size comparison this next is an older 6" M586:
I'm no expert on barrel interchangeability but the old Dan Wesson revolvers were designed to do just that and often came with 3 different barrels that could be unscrewed and changed. I think you need a gunsmith to remove these S&W barrels, you don't just pop them off by unscrewing them BICBW.
There are two answers for the "reasons" for the different barrel lengths. The fun answer is "because". Because folks like these guns and collect them and want them.
But the technical answer is that they do have different uses but even that depends on the users. Technically, the shorter barrels are more easily concealed/easier to carry/easier to wield indoors as house guns or in the field. The longer barrels are more appropriate for hunting and long range shooting because bullets travel faster and farther from longer barrels. And they are more accurate, especially at long distances.
OTOH, you'll get comments that differ from mine from so many lovers of these guns that my advice is fasten your seat belt and pay attention to all of the comments because you'll learn something every time. There are some real experts on the Forum. I'm just a journeyman, lifetime shooter and lover of all things in the world of firearms, mostly.
