I replaced the 7 round unfluted cylinder in my 5" 686-6 with a 6 round unfluted cylinder I got from Midway USA.
Smith & Wesson Cylinder Assembly S&W L-Frame Model 686 Unfluted
When I last looked, the unfluted cylinders were out of stock, but they still have the fluted 6 round cylinder in stock.
Maybe I got lucky, but this was an easy DYI project that was pretty much plug and play gunsmithing with Legos, at least in my case.
The Midway cylinder assembly came with an extractor already cut for proper timing. I removed the extractor rod, extractor rod collar, extractor rod spring, center pin, and center pin spring from original cylinder assembly, and then installed them in the new cylinder assembly.
Remember, the extractor rod has reverse threading and is screwed into the extractor counterclockwise (opposite of the usual righty/tighty/lefty/loosy which are the technical terms I think). You can order a special extractor rod tool online, but I found a pair of pliers and a relatively soft piece of metal like aluminum or brass works for me. Just wrap it around the knurled end of the extractor rod and grip it firmly with your pliers. Don't go all Hulk on the rod or you might deform it. I also used empty shell casings or snap caps to support the extractor in the cylinder when removing or tightening the extractor rod.
Once I got everything back together, I checked that end shake and b/c gap were good. Carry up and cylinder lock up were smooth and positive. I used a Brownell's Service range rod to check barrel and cylinder alignment which also checked good. I've run about 100 rounds of 38 and 357 through the gun with no problems.
As a side note, I actually installed the original 7 round cylinder assembly in my 6 round 4" 686. "Why didn't you just swap the 2 cylinders?" you ask. Good question - kind of wondering about the answer myself. Thinking about sending the fluted 6 round cylinder to Pinnacle for a 9MM conversion. (I've REALLY got to get a cheaper hobby like knitting.)
The 7 round cylinder was a tight fit and didn't rotate freely in the 4" frame although it had worked fine in the 5" 686 Plus frame. The other 2 cylinders spun free in both frames without any excessive end shake or b/c gap. Maybe it was just a case of tolerance stacking.
It was an easy fix though. I used a micrometer to measure the 3 cylinders I had, from the front of the cylinders to the round extractor bearing surface on the back of each cylinder. The two 6 round cylinders measured exactly the same. The 7 round cylinder was just a couple thousandths of an inch longer. I used the old file and fit method to take off as little metal as possible. I'd make a couple of passes on the round bearing surface with a fine India stone and then check the fit. It only took a couple of tries until the cylinder spun without resistance. What little metal I did take off did not seem to affect end shake or b/c gap as far as I could measure with my feeler gauges.
I didn't have to do any kind of work on the extractor star. Carry up and lock up were good with the new cylinder. If you get any resistance when cocking the hammer, you might have to dress the edges of the extractor star that the cylinder hand slides by when the cylinder locks into place. A number of years ago. I had to fit an extractor star when I installed a 940 9MM cylinder in a Model 60. If you look at post #22 in the following link, you can see what I did. It's a J frame but the principles are the same. And since the extractor in the Midway cylinder assembly is all ready cut, it would probably take only a minimum of metal removal to eliminate that resistance. Again though, take off just a little at a time, checking function in between.
Another "9"60
If you decide to do it yourself, hopefully it will be an easy swap like mine was. My out of pocket cost was about $120 total. If you don't feel like you can do this yourself, just looking at 8shot's post, costs for sending your gun to S&W might add up to the point that it might be better to sell your 686 Plus and buy a 6 round 686. Or even better, keep your 686 Plus and then look around for a good deal for a used 6 round gun. What was that really old Doublemint chewing gum commercial, double your pleasure, double your fun?
Good luck,
Pete
