In my Uberti-made reproduction of the Winchester 1873, I use the following:
Winchester case
Winchester Large Pistol primers (the .44-40 case is made for pistol primers, NOT rifle primers).
200 gr. flatpoint bullet, .429 inch diameter, cast fairly hard.
HP-38 powder - 6.8 grains.
About 1,100 feet per second from a 24-inch barrel.
I have not had good accuracy with Goex FFFG black powder loads I have tried. Even with a soft lead bullet and lubricant designed for black powder, accuracy has been about 5 or 6 inch groups at 50 yards from a benchrest.
I've learned from others that this is not uncommon. the Italian made .44-40s tend to have larger bore sizes and shallow rifling, the latter not being conducive to accuracy with black powder.
Best accuracy has been with jacketed bullets. The Remington factory load I fired would put five shots into a 2-1/2 inch circle at 50 yards. The rifle can do better; but my eyes cannot.
As a plinker, it's a great rifle. Not bad as a home defense rifle, either. That flatpoint .44-caliber bullet packs a lot of wallop.
Remember to determine the longest cartridge length with which your rifle will reliably function, then make your rounds a few 10-thousandths of an inch shorter than that maximum. An overly long cartridge will tie up the works!
Have fun!