I've explained this before, but since there seems to be several members here newly investigating the topic, I think it bears repeating: I think the whole lock issue is an overblown "tempest in a teapot."
First and foremost, I simply cannot accept the word of anonymous internet posters as authoritative. Even if we assume that everyone reporting "auto lock" is 100% truthful (and I don't think they all are), we still have the issues of unknown expertise in diagnosing the problem and, as described by another member (I can't remember who or I'd give credit), the "internet echo chamber.
As James K and others have pointed out there are a myriad of other issues that could cause a S&W revolver to lock up. The only way I'd be willing to assume that the dreaded internal lock was the culprit of the problem without pulling the sideplate would be if I could see the lock "flag" protruding upward and that would not be possible with shrouded and internal hammer models. I'd be willing to bet that a large percentage of the "auto locks" reported are actually caused by some other issue, but due to the rather vocal complaints on various forums, the lock is assumed to be the culprit.
Secondly, I reject the notion that the design of the lock, at least on the newer specimens that I've examined, is conducive to "auto locking" unless QC issues like defective or improperly designed parts are also present. The manner in which the lock works is actually quite simple: the "flag" has a small protrusion which, when the "flag" is rotated up, engages a groove in the hammer and prevents its movement. Because the "flag" is rotated up and back to engage the lock, it will be pushed forward and down and thus out of engagement when the revolver is in recoil. Also, when the hammer is fully forward as it would be when the revolver is fired, the "flag" protrusion is not lined up with the groove in the hammer and thus the lock cannot engage. The fact that the majority of reports of "auto lock" seem to be with lighweight guns in hard recoiling calibers like .357 or .44 Magnum, which place more strain on all the parts, and guns with relatively low round counts further reinforces this notion. Also, I don't believe I've ever heard/read a report of "auto lock" reoccurring after the gun was returned to S&W for repair.
Finally, we have the "internet echo chamber" to contend with. Because forum posting is largely anonymous, it is not only possible, but likely, for one incident to be reported multiple times and appear to be multiple incidents. For example, if one person experiences an auto lock, two other people witness it, and all three post of three forums under different handles, one incident would suddenly appear to be nine incidents. When you then take those nine reports and start repeating them as "I heard about this guy who had an auto lock" as is often done, and the one original incident may very well appear to be dozens, if not hundreds.
