The common non-gun causes of early slide stop lever engagement are shooter-induced (nudging/bumping slide stop lever) and ammunition (fat bullet profile hitting the lever's tab).
The most common mechanical causes of the slide stop lever engaging prematurely in S&W 3rd gen guns is either a damaged slide stop lever or side plate.
I'd check the guide rod plunger's tension, but the more common problems are the slide stop lever assembly or the side plate being damaged.
A damaged slide stop lever can occur when the plunger and/or its spring no longer exert the necessary tension against the angled portion of the side plate, or when the angle between the lever body and the lever assembly's pin (shaft) pin decreases and allows the lever's tab to reach farther into the magazine and contact the noses of the rising rounds in the mag body. (Recoil forces can act on the slide stop lever and sometimes eventually cause it to acquire a slight inward or
A damaged side plate can occur when the 'legs' (or prongs) of the side plate which secure and anchor it on the head of the sear pin become loosened, bent or even broken off. This can allow improper movement at the top/front of the side plate where the small angled section of the side plate sticks out above the front of the left grip panel and engages the slide stop lever's plunger. If the angle changes it can lessen the tension between the parts and allow the lever to float upward under recoil (and it can also trap the lever's plunger under the bottom edge of the plate if it shifts upward too much).
Improperly installing a grip can damage the side plate. If the person isn't paying proper attention and allows the corner of the grip to snag and catch the side plate, and then applies force to seat the grip on the frame, it can damage the legs. This improper leverage can result in the rear leg (or prong) of the side plate becoming tweaked and slipping over the end of the sear pin, becoming bent and spread too wide in relation to the other leg, or even being snapped off.
If gentle pressure from a finger tip & thumb can wiggle the small angled plate (which sticks out from the front of the left grip) up & down, generally either the side plate's legs aren't securely snapped over the recessed left end of the sear pin, or else the plate is damaged.
No way to diagnose what's happening with your gun, though, unless someone looks at it who's familiar with maintaining and repairing S&W 3rd gen guns.
The good news is that both parts are relatively easy to replace (although replacing the side plate requires knowledge of stripping the frame since the hammer, ejector and firing pin safety lever are held in frame on the side plate's pin).
Both parts still have to be checked for normal fit and function in any particular gun, of course. Every once in a while it's possible to find a slide stop lever that may not drop into a particular gun. The gap between the back of the slide stop lever body and the outside of the frame should be uniform (not narrower at one end or the other). I'm told they use what's called a 'lollipop' gauge to check for this gap during production, but armorers are told to simply look for uniformity of the gap, good function when doing bench checks with dummy rounds and then normal functioning out on the firing line.
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Ret LE Firearms inst & armorer
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