I have acquired a S&W Model 18 .22 Combat Masterpiece. Fires flawlessly in single action, but will not reliably discharge in DA. Adjustment screw on the main spring is turned all the way in. New spring needed?
New spring is one choice.
If adjusting screw has been shortened, that is a choice.
Is it really clean on the inside? A binding effect from gunk can slow down the DA hammer movement.
Bad DA timing from worn parts could be a choice.
There are several options and others will think of more.
Eliminate one failure possibility at a time, or take it to a good gun smith. [email protected]
As a Police Firearms Instructor whos Department used mostly officer owned Smith & Wesson Revolvers from when I joined in 1974 until the dept issued Sig P228s in 1992.
I cringe when I hear the S&W Strain Screw called an adjustment or adjusting screw. That screw is a certain size for a reason; To put a proper strain on the Mainspring so that the revolver will fire when needed. Yes I have found revolvers that did not work and found the strain screw backed out or had been destroyed with a file. Playing with the strain is something we hope only the bad guys do.
Jim, I often wonder...... why did S&W design the revolvers with a strain screw in the first place. The main spring could have been designed to have a fixed amount of tension, by moving the location of the slot in the bottom of the grip frame forward a little, to create the proper tension in the spring. Instead....they made it adjustable with the screw. Bob
I tend to agree with Bob on this one. It would have been too easy just to machine a fixed slot in the frame for the spring. Loosening the screw is obviously the wrong thing to do because one can never predict when the screw might back out more. On the other hand, shortening the screw, I feel, is a perfectly viable method of reducing spring tension. With these two caveats. After shortening the screw, test fire the revolver with a good mix of ammo to insure 100% reliable ignition. Second, over time the spring will lose some tension all on it's lonesome, so periodic testing, again with a good mix, of ammo is required. Assuming the revolver was 100% reliable when the screw was initially shortened and then after some point FTF begin. The choices are simple, replace the spring with one that is the same as the one originally. Or get a new strain screw. The problem occurs when Bubba does a poor job with the screw to start with and the revolver never was 100% reliable. Or the gun has fallen into the hands of someone that doesn't know how to deal with FTF in a revolver that may have had a strain screw shortened. Too often revolvers with ignition problems find their way onto the secondary market because the owner doesn't know how or want to deal with the problem.
Engineering Axium: A spring will lose 10% of it's tension in it's 1st 1% of life. Then the spring tension will lose small amounts of tension (over time or cycles) linearly until failure.
I would clean the gun, thoroughly (inside and out) first, then, go from there. 22 ammo is not always the cleanest to shoot. I like (Rdr)Bill's observations and, based on same, would address the most simple explanation/fix, initially, and persue the fix from there.
I also think Rdr Bill is right on the mark. If you think the screw is too short a cheap way to test it is shim between the screw & spring w/ a bit of leather or cardboard. THIS ISN'T A PERMANENT FIX, only a test. If it works you need a longer screw, if it doesn't you have other problems.