ok the rim usually isn't even under the extractor yet when it jams. just barely underneath it
"... isn't under the extractor YET"?
If the round has been properly fed from the magazine it should be FULLY under the extractor and positioned held against the breechface as the slide goes forward. If the slide is moving forward and the round isn't YET under the extractor, something is wrong. The first thing that comes to mind is that
the mag spring has weakened, and isn't pushing the round up far enough or fast enough to be properly positioned for the rest of the feed process. If that's the case, the bottom of the breechface would catch the top of the round's rim as the slide moves forward and that could cause the problem you describe...
Does it do this with all mags, or just one? If you're not sure, you need to find out... if only to know whether you need just one mag spring or several.
You may have to order a new mag springs from Ruger; if you're lucky, they might replace the springs under warranty. It won't be expensive if you have to pay for them. Wolff (gunsprings.com) doesn't seem to carry springs for the SR9 or SR40 series. If replacing the mag springs doesn't work it may be time to send it back to RUGER for a check... I'll bet its bad mag springs.
Note: while many folks feel that ONLY working (compressing springs and releasing them) causes spring problems (wear),
leaving a mag fully loaded and stored for long periods can degrade some mag springs, too. Wollf warns against this on their web site (in the FAQ area) with hi-caps in particular (or smaller mags that hold more rounds than usual.) It all depends on the design of the mag and whether the springs, when the mag is fully loaded, has pushed that spring to it's elastic limit.
Springs in loaded 7-round 1911 mags, for example, aren't near their elastic limit and can be left loaded for generations. Springs in some of the new 17-18 round mags will sometimes fail relatively quickly if the mags are left fully loaded for long periods. If you've got the 15-round mags in your gun, those mags are the equivalent of a hi-cap mag -- that's a LOT of rounds for such a small mag -- and some full-size .40 guns don't hold THAT MANY rounds.