for a load with a 180 gr cast bullet in the 38-40 winchester cartridge using "Titegroup" powder & a standard primer:
start with 5 grains & work up to.... a MAX of 6.4 grains.
velocities listed from 819 to 990 FPS.
also, keep your MINIMUM OAL at 1.600.
pressure listed at 13,300 PSI at max load.
this info is from the Lee reloading manual, 2'nd edition.....
Sheriff Jim Wilson writer of "shooting times magazine" found his most accurite load in this calibre using Titegroup powder, shooting a colt single action army with the 5-1/2" barrel. he was using 6.0 grs of titegroup. at 25 yds he was getting 1-1/2" groups. who knows, it may be a good load in your rifle.
also, you should get more speed from the rifle length barrel than the above listed velocities.....
if you can find the "Hodgdon's 2007 annual manual on reloading" at any good magazine stand it has a good article on these old Black powder cartridges including the 38-40.

the very next article is about "TRAIL BOSS" powder......
hope this helps, happy shootin, & let us know how the old classic shoots, when you can...
Ruffbird.