I just ran QL for RL 15.5 and was surprised to see at 87 grains of powder produced 2092 FPS at only 36700 PSI well below the 39,000 PMax
“Hot rodding” double rifle calibers is wrought with perils. The goal is to match the original regulation of the gun, not to glean additional velocity. The biggest issue with using modern powders in these calibers that were designed for stranded cordite is that they do not fill the case sufficiently. That in turn leaves us with a variety of stuffings from dacron, to rod backer, to wads, etc. Each of those stuffings creates a different pressure that cannot be calculated effectively by Quickload or extrapolation.
I know it wasn’t/isn’t your intention to do so, but just putting this out as a public service announcement. Sometimes people buy single shots in dangerous game rimmed calibers from 450-400 to 470NE. This in turn has them thinking of just how hot they can load the rounds since they aren’t worried about regulation. This is where dangerous thinking collides with powders that don’t fill the case requiring stuffing.