You don't mention if the bore is a .377 (traditional 38-55's were usually .377 or even greater) or a .375 (some newer rifles).
FWIW, I have a 38-55 Marlin. This rifle has the .375 bore and after experimentation with bullet, powder, and primer variations found that 39.0 gr of AA-BR2015 with the Hornady 220 and Win rifle primer give this rifle a consistent MOA at 100 yds.
I went through a variety of powders including Hodgdon 335, IMR 3031, RL 12 and others in the general fast-medium range. I also tried various cast and other jacketed sizes (some spitzer's as single shot loads) but for this rifle the consistency in accuracy was the above combination.
So, if your experience is the same as mine, enjoy your experimentation. The 38-55 was one of the favorite old Schuetzen medium range cartridges with fine accuracy and was and is a fine deer cartridge. As an aside, I tried BR2015 in my single shot 32-40 and found it just OK with BL-C2 as the better for accuracy.