Some couple of weeks ago I was challenged on what I'd presumed was common knowledge that the pressure limit for the trapdoor action was 28K PSI. I've been working on this assumption for years. Now, I don't have a Trapdoor but I do have a Contender and it was generally conceded that the Contender should be limited to the same pressures as the Trapdoor. Interestingly, that 28K PSI translates to the maximum breach thrust permissible in the Contender action.

Anyway, I've been researching this and found that several loading manuals recommend 28K PSI as about the max for the trapdoor action (and by extension, the Contender). First is the IMR Smokeless Powder Reloader's Guide. My copy is the revised in 2005 and shows 28K PSI and 27,400 CUP as the maximums for the Trapdoor. Interestingly, both measurement systems are shown. I believe that the newer data is shown in PSI and the older in CUP but I've not been able to verify that yet.

Ken Waters states in Pet Loads that 18K CUP is the pressure to which factory ammo is reportedly loaded. I've read elsewhere that the max average pressure to which factory ammo is loaded is 21K CUP

Speer #11 says that the industry standard is 28K CUP.

The TC Load Book shows no pressure info but does show the max charge of H322 with a 300 gr. Hornady FP to be 54.7 gr. This is about 18K CUP in the Hogdon manual. However, the Hogdon manual goes to 28K CUP for both Trapdoors and pistols (presumably Contenders).

Now the Sporting Arms and Ammunition Manufacturers Institute, Inc. (SAAMI for short) is the standard setter for ammunition. Now, if they say that a maximum average pressure of 28K PSI is the standard for the .45-70...

Although one should be circumspect about internet data (look at Wikpedia's recent brooha for proof of this), there are some well thought out sources which bear consideration. This RealGuns.com article is one that somebody interested in this subject just might consider.

My conclusion? That 28K CUP is a reasonable maximum average pressure for the .45-70. I will remember that every firearm needs to be evaluated for safety PRIOR to use.