If the hammer and trigger are still free to return without binding,
@WrongHanded, might be right, though I think he's mixing up the locations of the sear, etc.
Check out the pic below. What I think
@WrongHanded is suggesting is that the hammer's DA sear is catching on the trigger nose as the hammer's falling, which might be caused by a DA sear that's too long.
What's confusing to me is why it "sometimes" does it. Maybe a worn/cracked hammer or trigger pin? That'd definitely be a trip to S&W.
If this (sear/nose interaction) is what's going on, I'm also confused on why DA function would be unaffected. Seems to me if the DA sear were this long, it'd hit the trigger nose after the hammer starts coming down as well, though I'd actually have to have an opened gun in front of me to look at clearances, etc.
If the hammer and trigger bind up rather than freely returning, I tend to think the SA sear is binding against the trigger cam as the hammer's falling. I just described that in
a recent thread as a faux half-cock, something S&W's are definitely not engineered to do.
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