there is a 45/120 factory load in germany from WM Bullets with CIP proof.
the 485grain lead bullet makes 580 m/sec with an energie of 5434 Joule. Wolfgang Romey's 450/400 3 1/4" factory load have only 5308 Joule.
Because the 450/120 have the very strong CIP proof I have to believe that it is save for any old Blackpowder gun.
Two years ago I have seen an american made double gun for the 450/120 in the net. it was only a old 12 bore sxs with two . 45 barrel liner.
I cant imagine that the 50/140 cartridge is able to do the same.