Soft cast flatbase bullets work. The Keith Style SWC or RNFP work equally well in everything I've tried them in which includes numerous MKI/II/III .455 and 45acp webley revolvers.
THE SECRET to making a webley MKI/II/III shoot well is to use very soft cast bullets and use a rather "stout" charge.
Your way, ^, has yielded best results to date over here as well, jet.
AR brass, 2400 powder(thanks for that tip way back when Dingus), heavy crimp with mag primer, worked up with the chonograph to 700-750fps, 230gr dead soft cast .454" Keith SWC.
Does not solve those chronic high and left POI's I struggle with, but in the hands of other, more competent, pistol shooters, she'll
snuggle 'em in there to poa at 15yds.
Just ignore that trough of unburned powder laying at 6 oclock in the bore. 'Tis dirty stuff at these low pressures.
4.5-5 grains of unique does just about as well, but, is not as idiot proof at the loading bench as 2400 is, and this 'ol man loves the old girls way too much to take
any chances with their health.
The 2400 charges need to be somewhat "stout" as jet mentioned, in order to work. Enough powder in there to equal or slightly exceed the original BP vel. spec's. Win's point as to the cyl. gap affecting speeds is solid advice as well.
As has been repeated by Dingus, 2400 is better suited to the larger capacity carts., and will disappoint if downloaded to mild and slow in the smaller cases.
The .44spl. RR, it just loves the stuff.