Recently purchased a 629-4 with 3" barrel, weapon in good condition but had very late carry up. Replaced hand, fitted reduced power mainspring and rebound springs. Function checks were initially OK but occasionally during dry firing, had trigger jams at the start of the DA stroke. Manipulating bolt release frees trigger up and weapon then cycles normally. Weapon is clean, no apparent mechanical damage. Bolt, rebound slide & spring, ejector rod, cyl gap, headspace all appear to be in correct configuration.

Possible causes?

Have you tried putting the OEM springs back in? Could also be an issue with the newly configured hand, excessive endshake (too much headpace).....actually, any number of things. Also, try dry firing with empty brass in the chambers.

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Ret. LE, FA Instr, S&W Armorer

Endshake is barely perceptible, less than .001". Barrel-cyl gap is within specs, headspace not an issue as weapon jams with no cartridges in charge holes. Cyl stop and hand action normal. Sear and trigger engagements look correct. Suspect the reduced power recoil spring as a possible source of the problem. Will continue testing.

Try dry firing with the cylinder open, while holding the thumbpiece back. This will eliminate the cylinder assembly from consideration in the issue. If you're using a reduced power rebound spring, the trigger may not be returning fully forward to properly reset.

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Ret. LE, FA Instr, S&W Armorer

Not that this could be a root cause, but have you populated the cylinders with either spent cases and/or snap caps before testing?

A previous owner may have replaced the hammer and failed to get the sear fitted correctly.a double action sear that is a whisker too long can lock the trigger up in the beginning of its stroke.

The trigger may not be pulling the cyl. stop down soon enough with the new hand. This would cause binding. If it's a part time thing, it could be a borderline case, where sometimes it just clears and sometimes not quite.

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protocalldesign.com

I’d pull the side plate, keep my thumb on the hand so it can’t pop out and carefully cycle the action to see what’s happening.

My money is on the rebound spring.

What happened to the OP?

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Ret. LE, FA Instr, S&W Armorer

I had this happen when playing with reduced power springs in my 629-4. Long story short, I put the OEM springs back in the gun and it's been fine ever since.

This FTW!

Seen it many times.