Had my 990L to the range today and it shot great. Really getting used to the trigger and shot my best groups of the day with this gun (even better than with my BHP, which surprised the heck out of me).

But when I got home this evening and went to do a little cleaning...took the slide off, mucked out the inside, put slide back on and hmmmm....rear sight seemed loose. I could slide it a bunch (for a rear sight), left to right, within the dovetail.

Wondering if I whacked it or something....and when I went to center it again with the little screw on the right side...it would center back in place (but the screw would stick out a bunch) but it would still move side to side...and it felt like it was sliding over a ratchet or gear or something that would make it click.

There seems to be a detent pin under the right side of the sight, just under the screw head, but can't see it from inside the slide. Have no idea what it's for....will have to do more research I guess.

I claim no expertise or even much experience with replacing sights. And I can't for the life of me figure out what I did or how to fix it. Unless someone has a suggestion or three here, gonna be a trip to the gunsmith.

Damn, I really like that gun.

The detent pin is spring loaded and is what holds the rear sight to the slide. Push it down with a small screwdriver and the sight should slide off to the left. The sight adjustment screw will uninstall clockwise and install counter-clockwise.

As far as sight installation goes, the SW99/P99 system does not get much easier.

Perhaps fate is telling you to get night sights.

I have the same issue with my night sights. The pin is split at the top and sticking. I dont get how this will securly hold the sight when it is working. Theres nothing to prevent the sight from sliding to the right.

My local dealer tried ordering a new pin but it never came.

The old-style 99 series rear sight base is held in position via a spring-loaded plunger that has a machined slot in the top. The slot holds the edge of the 4-sided windage adjustment screw. The plunger is under tension from a spring. This method of holding the sight base positioned in the slide's doevtail can result in some minor "looseness" of the sight base, depending on the fit of the edge of the windage screw inside the machined slot in the plunger head. Normal.

If someone (previous owner?) has pushed down on the plunger and let the sight base slide to the right, over the head of the plunger, the plunger has become trapped underneath the sight base. If this has happened, the windage screw must be removed in order to remove the sight base.

Two things to keep in mind when removing the rear sight base are that the plunger and its spring are under tension (cover the sight to catch them), and the screw has a left hand thread.

Once the windage screw is put back in the sight base, the base is installed in the slide's dovetail left-to-right, while the plunger is held down with the flat side of a slot screwdriver until it can capture the edge of the windage screw in the plunger's slot. The machined slot in the plunger head is aligned parallel to the slide, so a flat edge of the screw fits within the slot once the sight base has been moved far enough to the right to capture the plunger. (Wearing eye protection is a good idea, and take care not to let the plunger fly off to parts unknown if the screwdriver slips off the plunger head.)

Note that the slot machined in the head of the windage adjustment screw has a unique bevel, and finding the right slot screwdriver can be interesting.

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Ret LE Firearms inst & armorer

If you lose parts and cannot find them, you might try Walther or Earl's Repair Service, as I think the S&W 99 and the 990L use the same sight parts as the Walther P99, 990QA, and the 990. Fastbolt, please correct me if I am wrong on this.

My pin is broken. I found a picture of on and as you said is a groove. However mine has only one of the sides of the groove. Securing the pin to the left but not the right.

Call S&W yourself - they may have the part. The part number is 264190000.
The Walther part number is 2627744.

The 99's using the plastic rear sight use the same plunger & spring, last I checked the parts lists.

The newer 99's may have plastic or steel rear sights (PPQ's came with steel, last I looked). The new design steel rear sight uses a clamping sleeve, slipped underneath/inside the sight base (and a sight pusher is recommended to install & drift adjust the rear sight). The clamping sleeve is replaced if the rear sight is removed.

The sides of the machined groove in the top of the plunger can be broken if the rear sight takes a hard enough impact. I've had to replace at least a couple of them on guns which were dropped or suffered hard impacts on the rear sights, and one side of the groove snapped off.

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Ret LE Firearms inst & armorer

Last time I called S&W they were out of plungers, they refereed me to Walther and they were out also. I ended up using a key file to file the groove deeper and put a strip of business card under the sight to add resistance. It worked well enough I forgot to order the new plunger. If you find the plungers get a spare, they are very fragile.

I own the SW99 compact in .40. It was the first handgun I owned and I still love it. I did notice that the rear sight was drifting slightly but was able to straighten it out by turning the windage screw. I'm currently looking to upgrade to night sights and I was wondering if anyone had any recommendations. Do they sell sights that do away with the windage screw. A fixed sight in other words. Im hoping someone will have experience with a good set and could point me in the right direction. Thank you all!!