s&w 66-fails to fire in doubla action pull?

[spispeza]

i have a stainless finish s&w 66 k frame 357 mag 4inch barrel.the problem im having is it fails to fire when you pull the trigger in double action mode.when the hammer is down.not pulled back.it fires all the time when maually pull the hammer back in single action mode.i took the grips off .the screw that i think controls trigger pull weight was backed out some.so i tightened it all the way.it still fails to fire in double action mode most of the time.im using win,federal,rem.,PMC factory 38/357 ammo.no reloads.
ive fired other revolvers in the past in double action mode,never had a failure to fire.mentioning this due to a person saying it might be my technique in pulling the trigger.
i think some part is worn or needs adjusting.ive cleaned it as best as i can without disassembly.cylinder locks up ok.thanks

[doorgunner]

The screw that you are referring to is the strain screw.  It should be very snug, not just tight.  It preloads the hammer spring, and is vital to proper ignition.  Put a bit of tourqe on it, when you tighten it.  If that doesn't fix your problem, then take a look at the firing pin.  If it's fully intact, try various loads.  If the problem persists, send it back to S&W.  Their customer service is great, and they will make it right, most likely at no expense to you.

[dubber123]

If the gun has been shot significantly, it may just need a new mainspring.( cheap and easy to replace).  My old model 10 worked fine for may thousands of rounds, ( 35,000) in one year), and it was used when I got it.  It eventually started to misfire in double action, but worked fine in single action.  I swapped in a fresher, but not new mainspring I had, and the problem disappeared.  A Smith delivers quite a bit heavier blow to the primer when the hammer is cocked versus firing in double action.  I think you will find it is something simple like a spring, good luck.

[PaulS]

Yep, It either needs to have the main spring tension increased or a new one. This is a common problem with all guns that use a flat spring. It is a good reason to use coil springs for main springs.

[dogwhisperer5]

   Most likely a fired large primer (pistol or rifle) with the anvil removed placed over the end of the strain screw to fit between the screw and the mainspring will fix your problem. It worked for me on a 4" 66 with a problem igniteing hard primers.