S&W made the 990L series a bit differently than Walther made their roughly equivalent model, which is the Quick Action (QA).
If you don't have an owner's manual, download the .pdf online manual from the S&W website and read about how the 990L model is operated.
http://www.smith-wesson.com/wcsstore...other/SW99.pdf
S&W basically used the QA sear housing block in the 990L's frame, but didn't include the decocking button in their slide design. That means the 990L has to be 'decocked' in the same manner as the Glock (or S&W's Sigma), which involves pulling the trigger on an empty chamber to begin the field-stripping process. Why? Nobody seems to claim to know why that decision was made.
The 990L series was pushed to LE while the M&P pistol project was being developed. The best explanation I heard offered was that the QA's (990L) consistent trigger pull was considered more marketable to LE customers (no decocking to return a gun from a single action/cocked condition to a double action only condition with a heavier trigger stroke). The QA's trigger stroke was lighter than the full DA trigger stroke by about 3 pounds (on paper, anyway).
I ordered one of the last SW99 9mm compacts just before they switched over to assembling only the 990L's (although they'd still make the standard SW99's for LE customers currently using the SW99's, if pressed). I didn't care for the trigger of the 990L series, myself, but that was just a matter of personal preference.
FWIW, the striker spring used in the 990L (and QA) is heavier than that used in the standard SW99 (and AS), and a different trigger bar guide is typically used in the sear housing block, as well.
I've been to a few SW99/P99 armorer classes and own a pair of SW99's. I also carried an issued SW99 .40 for a while, too.
Although the premature slide stop engagement issues were resolved in the .40 99's, and some further refinements and revisions have occurred in both the SW99/990L and P99 lines, I still happen to prefer the 99 series chambered in 9mm, myself.
I didn't care for the SW9945 I tried for a while. Not that anything was wrong with it, or it exhibited any functioning issues, it's just that it didn't seem to lend itself well to a reasonably sized .45 pistol, well-balanced pistol. Maybe if they had made a compact version ... but the M&P was apparently on the horizon as a new project.
I happen to think the 99 series is a design which never received the appreciation it deserved in this country. Maybe because Walther didn't really push the P99 line all that much. Dunno.
While I have no plans to get rid of my SW9940 (full-size), I don't enjoy shooting it as much as I do my SW999c (compact). That's a dandy little 9mm gun. I can actually wring some better practical accuracy out of it than I can my favored 3913. Of course, having shot more than 10,000 rounds through it I've become fairly familiar with it by now, too.
Nice little gun.
The triggers on all of the SW99's I've ever handled & shot seemed to improve with time, too.
I've posted a number of threads involving my thoughts on the 99 series in previous threads, so i won't get into any more details in your current thread.
Congrats.