All of which is moot ... the O P mentioned that his rifle ... the one he asked questions about ... is a Uberti HIWALL chambered in 45 Colt.
No worries as the Miroku Japchester with equal metal supporting the heavy breech block ... is chambered in 375 H&H mag.
I would go out on a limb and say that loading a 45 Colt will NEVER get to the point of pressure and thrust of the old Holland n Holland round.
Also please know that under the 255 grain boolit, 9 grains of Unique is the equal of the old Colt black powder loads.
Wanna get your eyes opened concerning the beloved 45 Colt round ... here is what to do ... load 40 grains of Ol E or Swiss 3Fg black powder under your 255 grain boolit. Fire this in your revolver and report back with your impression of the ORIGINAL 1873 loads for the 73 SAA Colt revolver. There is good reason for the US Army to request that they be toned down a little for the troops.
That 40 grain load is not for the faint of heart ... I assure you.