"Trapdoor" means different things to different people - and to different manual publishers. See below, then tell me what "Trapdoor" pressure really is.
Data Source...Trapdoor...Marlin...Ruger (CUP)
Lyman........18,000....28,000....40,000
Speer.........21,000....28,000....35,000
Hornady.....25,000....40,000....50,000
Hodgdon....28,000....40,000....50,000
Factory......18,000 reported