Postman, you are correct in every way; that is why actions & barrels are proofed in the first place!
In no circumstance am I suggesting using a low pressure, yet full NE load in an un-proofed (for Nitro) BP rifle!
Considering that if the rifle in question has the correct proof mark stampings, then what is wrong with formulating a typical NE load that runs at a lower pressure than conventionally proofed if the owner so desires?
If we decide to run BP or even NFB loads in our rifles as well then that's what reloading is all about, learning & having a lot of fun in the process using older rifles - albeit safely!
93x64mm