If you want to do multiple calibers on a G19 (PF940C or PF940Cv1) you should get a slide for a G23 or G32 Gen 3. Or an aftermarket copy of the same. The reason is the exterior barrel dimensions. The G23 ( 40 S&W ) and the G32 ( 357 Sig ) use a barrel that is larger that the G19 ( 9mm Parrabellum )
So, a G32 and G23 barrel will not fit inside a G19 slide. They have a larger breech block and are too big to pass through the G19 slide. Now, aftermarket 9mm conversion barrels are readily available to allow G23 and G32 pistols to shoot 9mm. Which means if you build your pistol as a 40 S&W or 357 Sig, you can also shoot the 9mm by swapping the barrel with an aftermarket conversion barrel.
If you build the pistol using a G19 slide, you cannot later change to 40 S&W or 357 Sig... there just isn't a barrel to do that, since the wall thickness of a 40 cal barrel becomes too thin with the G19 exterior barrel dimensions (which is why Glock made the G23 and G32 larger in the first place.) With a G19 slide, you have to stay with the 9mm Parrabellum chambering. (or buy a second slide & barrel)
That's my advice to any new builder. Build the pistol as a 40 S&W or 357 Sig. The when you want to shoot cheaper 9mm, you swap the barrel, magazines, recoil spring, and ejector into the same slide. You can also change the extractor parts too, but I've found the 40 S&W parts do a pretty good job on a 9mm.
