i have sort of replaced my 2400 with RX 7 solely 'cause it was easier to get and long ago replaced Unique with Universal . 'cause it was a hell of a lot cleaner,but,I may have to switch back to Unique as I am not sure how the new Universal is going to play out in all of my intended applicationsI will soon be sitting on over 20# of 2400.
It is far and away my favorite cast bullet rifle powder.
I looked at the same thing but ignored the gun and looked at the pressures reported. I then did some extrapolating and decided it was a safe charge.
This is where the older Lyman manuals are a plus, they show so many powders that are ignored in the newer manuals.
the 86's and Marlins are a step above the old black powder guns.
of course a Ruger is above them.
anyway the old springfields show 17-K CUP as a max pressure.
the marlin and 86 data shows 21grs of 2400 with Dacron and a 420gr bullet as 17.7CUP.
this tells me 20grs with Dacron or 21 without is the top end load for the 73's.
now you might find a load that works fine enough for you below that.
but I would bet using Dacron and 18grs of 2400 is about as far down as your going to find 2400 burns acceptably using anything much lighter than 400grs of lead on top of it.
FWIW, for several years I held two CBA records for big bore military rifles. Both were set with a Trapdoor Springfield. The four 5-shot group at 100 yards was with the Lee HB 405 grain bullet and 24 grains of 2400, no filler and WW LR magnum primers. The aggregate of four 5-shot groups at 100 yards and four 5-shot groups at 200 yards was with a Hoch 510 grain bullet with 22.5 grains of 2400, no filler. These are both standard loads for Trapdoors and a RB in good conditions should be as strong. HTH, Ric