The gun needs to be checked by a gunsmith familiar with S&W pistols, a S&W armorer or the factory. You may have a couple of things going on at the same time. That can make diagnosis a bit tricky, and requires someone knowledgeable in the guns to examine it in person and identify the problem(s) that may exist.

Some thoughts on what has been posted so far in this thread ...

The failure-to-lock back on an empty mag ...

If the other mags are letting the slide lock back when they run empty, but the one that came with the gun isn't, it's possible the mag spring and/or follower that came with the used 'factory' mag need to be replaced. (Not a bad idea anytime a used mag is involved, anyway).

The erratic and inconsistent feeding ...

Failures-to-feed are typically caused by the ammunition; the magazine(s); a dirty chamber; a damaged extractor; a barrel with an overly long tab (rear of hood); or a recoil spring.

Are the frame rails in good condition, or did someone run the gun too long without sufficient lubrication?

While a failure-to-extract can be caused by a damaged extractor and/or a damaged or weakened extractor spring, a failure-to-feed may be caused by a problem at the "other end" of the extractor.

For example, maybe there's excessive gunk, fouling or even some bit of debris that's accumulated under the extractor tail, or within the spring, preventing the tail from pivoting inward and letting the hook move outward as intended during feeding. This can create problems with feeding "timing".

Former LE gun, right? No way to know how it was maintained or repaired (if necessary). If the extractor required replacement at some point, was the extractor fitted correctly? If it was over-filed and is too tight, you can get feeding failures. Was the correct extractor spring used if it was replaced? Too heavy and you can get feeding failures (because the extractor has too much tension). These issues can become complicated by unexpected ammunition tolerances might vary when it comes to case diameter or rim thickness.

Was the barrel replaced at some point? If so, was the tab checked for fit? Too long?

Are the frame camming surfaces in good condition? (Those are the angled surfaces in the frame, to each side of the barrel's feedramp, where the barrel lugs run up & down during cycling.) Are the top points of the those angled surfaces in the frame still within proper spec, so the barrel is release to return up and into battery correctly? It's critical that the top of them are in good condition for proper feeding & functioning. Some peening can normally occur in the 'middle' of the camming surfaces (alloy frames can experience more wear from the steel barrel lugs running up & down them than when a steel frame is involved), but if the top surfaces are damaged the frame is generally not salvageable. (I once had a 6906 alloy frame get pretty peened and worn in the middle of the frame cam surfaces as the round count was approaching over 40K rounds, but the top 'release points' were still good and the gun was running reliably.) This is something I'd leave to the factory to determine, myself.

It can become understandably frustrating to have one or more problems interfering with the normal operation of a gun. Especially one as normally reliable as the 3rd gen guns. However, guesswork can only make things more frustrating, and 'trial & error' attempts by someone not trained with diagnosing and correcting simple problems can make things even more frustrating. Not to mention maybe creating further problems.

Why not call the factory and inquire about having the feeding problem in a former LE trade-in gun be examined and corrected?

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Ret LE Firearms inst & armorer