The Baikal SPR 22 does not have a real world safe working limit of 28,000 PSI. The manual warning suggests the use of SAAMI compliant ammunition which is limited to 28,000 PSI. Ruger for the #1 and Marlin for the 1895 modern lever action rifle in their owner's manuals, for each rifle they make, specifies the use of "industry standard ammunition" for the caliber intended for that rifle, which again is 28,000 PSI for both these guns in 45-70. There is no acknowledgement whatsoever in any of the owner's manuals that one can use ammo not compliant with SAAMI. Nor is Hornady ammo in 45-70 not compliant with SAAMI pressures. You think Hornady doesn't know some YoBo isn't going to load some LeverEvolution 45-70 in Granddad's Trapdoor?
But the real question asked in this thread is "what is the true, safe working pressure of the SPR 22 in 45-70"? The easy answer is that it is a SAAMI-compliant 28,000 PSI. The real world answer is that after certain firearms have been shot with above SAAMI pressures for a period of time without dangerous effects, safety is proven thru use. Obviously the Ruger #1 and Marlin 1895 have been proven to be safe with pressures in the 45-70 well above SAAMI, the Marlin up to around 43,500 PSI and the Ruger at God knows what. Still, not a practice their respective owner's manual sanctions, nor do they approve of handloaded ammo or "non-standard ammo" (read Buffalo Bore or Corbon heavy here).
The three guns referred to are all chambered in other calibers safely which operate at higher pressures safely, giving us a view of "safe" operating pressures, irrespective of what the manual says. The .450 Marlin in the 1895 "proves', or at least gives evidence that heavier loads in the 45-70 Marlin are acceptable. The enormous variety of high pressure calibers chambered in the #1 give confidence the use of non-SAAMI compliant 45-70s are 'safe".
Now, as to the SPR 22/MR221 Baikal/Remington, we also know it is chambered in the 30-06 safely, which operates at around 60,000 PSI. We are then dealing with a modern, strongly constructed action on the Baikal. Any comparison with this action's strength and that of the 1873 Trapdoor is flawed, as is the suggestion the pressure limitations of the Trapdoor be the limit for the SPR 22. And in fact, there are various reports in the blogs and chats of ammo use in the SPR 22 well over the 28,000 PSI SAAMI ammo.
The limiting factor is the bolt face size of the 30-06 and much larger 45-70. I'm not up to the math involved, but Kifaruforms.net did this extremely well some years back on this very subject. Search "Allowable pressures in Sparton double rifle". Note that "Sparton" is misspelled and you should enter the search the same way. The upshot of the math is that the SPR 22 should and does have a max working pressure of around 43,000 PSI, or like the Marlin 1895 45-70. As a final check against this information, you might rebarrel a Springfield Trapdoor to 30-06 and see how that goes. I suggest a very long lanyard to fire the rifle tied to a tree. I'll look for the report on those who suggest the SPR 22 is to use only the same ammo as intended for the Trapdoor.
So what is the "safe" working pressure of the SPR 22? Surely we know SAAMI compliant 28,000 PSI factory ammo is. And I do not suggest anyone violate the warnings in the owner's manual of any rifle. I do suggest the information presented here is a roadmap to making your own decision.
Finally, the two SPR 22s I have, one each in 45-70 and 30-06 are 7.5 pound rifles. I think the real limitation for much shooting would be the recoil level of Buffalo Bore 420 grain "heavy" load, or similar loadings. That would snap one's head around with instant headache.