Is there a slide assembly in 40 S&W that will fit on this pistol? I realize that the mags are different, but is the mag well the same size? Like to make the P95 a dual caliber. Is there a way to make the P89 a dual caliber to 40 S&W? I already have a 30 Luger barrel for the P89. I found that a SR9c or SR40c is interchangeable by buying a parts kit and magazine. All swaps and works fine. No need to change the ejector. Just switch tops along with barrels and recoil springs plus use the right mag and you have a dual caliber pistol. Like to see my P95 and P89 set to use 40 S&W as well. Thanks for any help. Mainer.
I like multi-caliber firearms also, and have a P91 that I've used as the basis for my project. Based on personal experience with other models, I believe the P-series 10 & 11 round 40 caliber mags should fit a P95 (or P89) mag well.
A 40 cal top half (slide, barrel, recoil assembly) is the bigger problem. Ruger didn't produce a P-series 40 with the same slide/barrel length as the P95 - both the P95 & P93 were only available in 9mm. The P94 & P91 40 cal parts are longer (and would look rather odd installed on a P95 frame, assuming they even fit). More importantly, both lack the second frame/slide interface (lower front of slide) that Ruger used with the all-polymer P95 frame - going with a more powerful cartridge with less frame-to-slide contact would probably be unsafe. In addition, the P95DAO & P95DC use a trigger bar configuration & corresponding cut in the slide, that is different than earlier P-series pistols; only the P95 models with manual safety use the earlier design.
In, short, I don't think a simple parts swap will produce a safe & functional 40 caliber P95. A gunsmith might be able open up the breech face and front of a P95 slide to accept a Ruger P944 40 cal camblock barrel, but I'm not sure if the barrel/block/recoil guide rod interface geometry was the same on the P944 & P95, and the recoil springs would likely need to be beefed up as well. And in the end, you would be using a more powerful cartridge in an all-polymer frame which was likely not designed for it, and could be unsafe.
As much as I like the .40 S&W (my favorite pistol cartridge ;^), it might be better to look down on the power scale, rather than up, if you're using a P95 frame. Since you already shoot .30 Luger, you might consider getting a spare P95 barrel & recoil assembly, and having a gunsmith sleeve the 9mm barrel down to .30 Luger. You might need reduced power recoil springs (depending on your .30 Luger ammo), but that might be accomplished by simply removing a few coils from a spare factory assembly.
Also, you could look at 45 cal P97 parts. I have little experience with them (I think the trigger bar is similar to the P95DAO/DC configuration, and the barrel is very similar to a P90 barrel except for the camblock), and even if P97 parts happen to fit the P95 frame, you would have to load your own .45 ammo with an overall length short enough to feed through a 40 cal magazine in the P95 mag well. But it might be worth a look.
Your idea of converting a P89 to 40 caliber might be more practical. I have a P89 slide/barrel/recoil assembly that fit onto my P91 frame (with a very small modification to the slide to clear the 40 cal ejector), so it's possible a P91 top half might fit a P89 frame, if above serial number 304-70000 (IIRC, a P93/94 slide will NOT fit a P85/89/90/91 frame). I have better results with P85 9mm mags in my handgun with my P89 slide, so I'm not sure if the 40 cal mags might need any mods to clear the P91 slide on a P89 frame, or not. And dropping a larger barrel into a smaller caliber frame can sometimes produce enough clearance issues to prevent the slide moving completely to the rear (something I encountered fitting a P90 top half to the P91 frame).
If you feel like trying it, P91 parts kits show up on GunBroker once in a rare while, sometimes at reasonable prices:
Anyway, your P89 idea looks like it might be the more practical approach, but good luck with your project, whichever direction you go (or don't go :^)!
As always IMHO, FWIW, YMMV, I'm NOT a professional gunsmith, caveat emptor, safety first, etc.