This is something that gets repeatedly asked and discussed when someone buys an early 585/686 and becomes aware of a recall.
The issue was with a specific brand and loading of .357 Magnum ammo, even though it was not specifically called out by name by S&W, probably due to legal reasons:
1) It was Federal ammo (which is known for having the softest primer cups, and why Federal primers are the choice of Jerry Miculek (and others) for reliable ignition in the target hand loads he uses with the lightened actions on his competition revolvers).
2) The loading was 110gr bullets, at high velocity and pressure, than with standard weight loading.
The combination of the two resulted in reports of primer cup flow back into the the hammer nose bushing hole, causing the cylinder to completely bind up.
The number of incidences was not wide spread, but it came at a time when S&W LE handgun sales were beginning to feel the pressure from Glock (and semi-auto pistols) making inroads.
S&W had to go into damage control and nip a potential PR disaster in the bud by issuing the recall, rather than continue to have reliability issue trickling in from LE agencies, which is where the initial reports came from to begin with.
The language of the recall was quite clear:
Firearm Recalls and Safety Warnings- FirearmsID.com
These are some of the key points.
"use of ammunition, which does not conform to industry pressure specifications or is particularly fast burning. Recent developments in ammunition manufacture emphasize the production of .357 Magnum ammunition with increased velocity and greater primer sensitivity."
"Although there have been very few reported incidents of cylinder binding, in view of our concern for our customer's safety and the reliability of Smith & Wesson products in all circumstances, we issue the following warning:
In a situation where a failure to fire can be critical - such as law enforcement or personal protection - do not use .357 Magnum ammunition with an L-frame revolver bearing model numbers 581, 586, 681, 686 or 581-1, 586-1, 681-1, 686-1, 686CS-1 without an "M" over the model number until you have had the revolver modified."
Most owners never experienced an issue, they are largely unaware of the existence of the recall unless they're read about it in a forum.
I have 2 586's and 2 686's, only one of the 686's has been back for the "M" mod. I've had no issues in any of them.
