Mongo,
Sounds like the manufacturer discourages smokeless loads. Sometimes modern steel only marginally strengthens a design. I will point you to the manufacturers again (My attorney and liability carrier will drop me as a client if I do otherwise.).
Spear lists max pressure for Trapdoors and Sharps rifles at 21k psi. You can download the Spear online at
http://www.speer-bullets.com/pdf/ReloadingSupplementalDATA/4570_300.pdfIMR link as posted earlier lists 28k psi max for Trapdoor.
Hodgdon link as posted earlier lists 28k cup max for Trapdoor.
My Lyman and Sierra books were loaned to a friend who's contemplating reloading and wanted to study before he invests so can't check them.
The difference between cup and psi is they are different measuring systems for chamber pressure. They are not interchangeable or corollary. Pressure in a load is relative to the powder and components/coa etc. listed in the manual for that load in that particular manual whichever system is used. Different manuals may have seemingly contradictory data. Check several manuals and stick to the lower given max until you have the experience to know if you can or should go beyond to the other manuals data. You may have no need, the most accurate is usually not the hottest load.
Again, if not sure, call the manufacturer and use what they recommend. Do not change components. Few Smokeless powders are forgiving and pressure can and will jump to dangerous levels if you do not stay within the bounds the manufacturers give in their manuals.
Hope this points you in the safe direction.
BB