I should add that some years ago, (pre lock), S&W had a recall on the 686's. Supposedly they said the guns could lock up with hot loads, because the hole for the firing pin was oversize. This would allow material from the primer cup to "flow" into the area left open by the oversize hole, thereby tying up the cylinder of the gun.
I'm not sure of the serial number range that was included in the recall, but I'm sure you could E-Mail S&W for it. I never returned mine because I never experienced such a problem. I even tried to deliberately cause it by firing some old 110 grain Super-Vel loads I had from the 70's laying around. No dice. The gun ate them without a hitch. The primers were extremely flat, and showed some cratering and tooling marks , but no more than any other hot handload would. Super-Vel manufactured some of the hottest ammunition avaliable at the time, and was loaded to much higher pressures than even +P ammo today. So I figured if it ain't broke, why try to fix it. Most of the people I talked to about this said the problem was sporadic, and not all guns in the serial number range had the issue.