People neglect recoil & mag springs. For dedicated duty weapons S&W recommends its armorers replace them either every 5K rounds fired, or every fire years of serve, whichever occurs first. The use of +P or +P+ duty ammo might accelerate the spring wear, though.
The sear release lever and the extractor are considered "fitted" parts.
Checking the fit of the sear release lever & proper decocking "timing" used to be done by eye, manipulating the decocking levers, checking the point when the hammer decocked, and then snapping the hammer to complete the downward movement of the decocking lever, but in recent years S&W has provided its LE armorers with some numbered drill bits that are used for GO/NO-GO gauges (the non-cutting end is the "gauge) to check this timing during lever movement. The sear release lever itself is adjusted by filing the foot shorter, maintaining the correct angle, to the degree needed to achieve proper decocking timing as it displaces the sear nose forward (letting the hammer fall).
The fitting of extractors requires the use of the factory-provided GO/NO-GO bar gauge to check the reach of the hook into the breech face. Filing the adjustment pad of the extractors changes the reach of the hook.
The tension of the extractors are checked using a Force Dial gauge (with the appropriate extension to hook over the extractor hook), with the tension amount checked at deflection of the extractor. There's a recommended tension range for different calibers/models, and optional lighter/heaver springs available to assist the armorer, repair tech or gunsmith in achieving the required tension for any particular pistol.
Granted, in my last 3rd gen pistol armorer recert class there was a 5906TSW in which an extractor slipped into a slide, and without any filing was found to be within the recommended tolerance when the extractor bar gauge was used to check it. That's the exception to the rule, though, and we were told that it was more than likely due to the newest tolerances being used in manufacturing the components and parts. Of all the extractors I've had to replace in 3rd gen guns over the years I've been an armorer, I've had to file the adjustment tabs on the extractors in order to get them to meet the "GO" specs using the bar gauge.
The 5906's are robust, durable and reliable service 9's, but there are some repairs that are best left to someone who has the knowledge and experience.
Just my thoughts.
