Is there a 'well-known' formula for calculating the chamber pressure in our rifles that results in reasonable answers? Am thinking to re-chamber a Trapdoor to 38-56, which is the 45-70 cartridge necked down to 38cal  and would prefer not to do anything 'near the edge', so to speak. Am thinking that it is likely OK, but my understanding is the BN case will result in ~3000psi more pressure than a straight case, though am unsure if this is true when going down in bullet weight & diameter.

Does Quickload allow for BP?

The pressure rise I was speaking of is one of those "all things being equal" type of things. For instance, the 40-70 Sharps, a BN cartridge, generates a higher MV & pressure than does the 40-70SS with the same load. When the 45-70 was being developed in the Trapdoor back in the day, the testing uncovered the fact that the BN cartridges developed ~25fps higher muzzle velocity but at the higher pressures. The Army decided that the additional MV was not worth the additional pressure. Wish I knew which BN cartridge(s) were tested.

Am guessing that the 368gr (max weight I'll shoot) bullet being pushed by 60gr of 1 1/2F Swiss (max powder) will not be a pressure problem in the Trapdoor.

Went back and reread the part about the gov't testing and they made up both a BN and straight 45-70 cartridge to test the 45cal in the TD when they decided to move away from the 50-70. It was their 45-70BN (which I have found no details about) that gave 18,000psi compared to the 45-70Gov't at 16,700psi. 

Am thinking about adding a recoil lug either in front of the rear tang or between the barrel/receiver and a barrel-band in the Gemmer-Hawken build to help keep things well located.