I was intrigued by Fastbolt's reply. I have a 5906TSW with a BAWxxxx serial prefix that has a sharp, full-looking, stiffly-sprung extractor, yet fails to extract and double-feeds a lot. However, its barrel has the large "9mm" marking without the word "parabellum." Would it still be worthwhile to send it into S&W? It's chamber looks smooth, but I don't really know what I'm looking out for here.
This forum never ceases to amaze me.
If it were me, I'd have the extractor checked for proper tension. The only way to do that is to use the force dial gauge.
There are 3 different extractor springs which are listed as being applicable to the 5906TSW, with the "standard" (production) one being the #10087 spring. The others are increasingly heavier. (FWIW, in the case of the newer TSW's the same springs are used & available for the 4006 as the 5906, even though the .40's use a different extractor.)
As an example, I was brought a new production 4006TSW that was exhibiting some occasional failures-to-extract. While the extractor spring didn't "feel" out of the ordinary to my thumbnail pressure, when I pulled the slide and used the force dial gauge the tension was noticeably too light (falling below the recommended normal range of 4-8 lbs).
Replacing the spring with a brand new spring of similar type (#10087) didn't put the spring tension up within the normal range, which I thought reasonably eliminated the possibility of the original spring being out-of-spec or damaged.
The extractor specs were fine (checked with another gauge), so it seemed possible the extractor spring recess might have been milled just a bit on the generous end of things. One of the heavier springs (I forget which at this time) put the tension up within the recommended range and a lot of live-fire testing revealed the problem had been resolved.
Bear in mind the failures-to-extract were observed with a couple of shooters and good quality duty ammunition, so the shooter and ammunition issue seemed less likely at the beginning of my assessment. As it turned out, this was just one of those pretty infrequent guns that actually needed one of the optional heavier extractor springs in order to give it normal function.
If you're using fresh, good quality factory ammunition, and the barrel has been kept clean and free of debris (like a bit of brass case stuck to the chamber wall), I'd call S&W and ask to have a shipping label sent for the gun to be examined and the condition corrected. That way they can check the barrel, extractor & extractor spring.
The extractors are fitted parts, meaning that the hook might not be reaching as far as is needed into the breech face. If it essentially "pulls away" from the case rim during that critical "moment of mass" when the recoil force is occurring ... pivoting out from the case rim too early due to either not enough hook "reach" into the breech face, or a light extractor spring ... you can see failures-to-extract.
Another possible cause of failures-to-extract (double feeds) can be a damaged recoil spring (kinked, bent or worn out). Armorers are typically told to replace recoil (and mag) springs either every 5 years or every 5,000 rounds on duty guns. Sooner if +P & +P+ loads are used.
Just my thoughts.