I thought we'd already done this...
You may (or may not, depending on how you've run Quickload) be accounting for the reduced volume in a necked case. Same powder mass for a given load (for 'ideally' the same same power transfer to the bullet --maybe a tiny bit more efficient with necked rounds) with a smaller volume will result in higher pressures. 7.62x25 has, I believe, a higher powder load than 9mm already. So you'd be talking an already above-normal powder mass, crammed into a smaller case volume than even 9mm.
Pressures will have to rise above 9mm +P in all likelihood (+P isn't all that "+" in the first place, being like 10% IIRC). Now, this isn't necessarily a deal-breaker since 9mm isn't the hottest thing to ever grace a recoil-operated pistol, although it will be murder on you lovingly crafted brass. But there is one load factor you haven't accounted for in assuming that even a 9mm-pressure Luger round will stress an action the same way; the case neck.
The 9mm is effectively a straight wall cartridge, so the powder gasses thrust against the breachface, and against the bullet which drags on the rifling; no significant axial load applied to the locking surfaces apart that due to the acceleration of the slide under bolt thrust (i.e. the breech trying to blow open). Put a significant neck in the chamber, and now your elevated pressures have another surface to bear down on in the axial direction. Both the bolt thrust as well as barrel thrust will contribute to locking lug loading. Again, it's not like 357SIG hasn't been shown to work just dandy in a lot of guns, but it is a design factor you need to account for & show green on.
Because you are chasing three things simultaneously that all push the design closer to failure; increased powder volume*, decreased case volume, and a necked chamber.
TCB
*I keep referring to powder volume since 9mm is so efficient that I doubt you will get a 30gr pill to Tokarev velocities without more chemical energy to work with. The efficiency gain in necking 9mm won't be detectable, so to bridge the power gap between 9mm and x25 more powder will be needed. Simply increasing the peak pressure won't help a ton outside of a super-short barrel case, since no Luger or Parabellum case will be significantly overbore with a normal powder charge; the pressure curve will simply spike more harshly without much more more area beneath it (the energy transferred). If you've run a souped-up Luger in Quickload or something similar to arrive at your conclusions, I'd love to be proven wrong; but since I don't have QL I have to SWAG it with a hazy memory of chemistry & thermodynamics.
