I’ve got a 100+ yr old .44-40 and thinking of reloading some rounds for it. I’ve read various reports of what powder and bullets are best to use. Just wanted to see if anyone here has some real world experience.

I have an old Win 92 (made in 1894) 38-40 that I reload for. I use Trail Boss with whatever lead FN bullets I can find.

I’m thinking that a Lyman reloading book will have all the info you need.

.44-40 is tricky because the groove diameter and chambers can vary so much between guns. You'll need a CAST bullet between .425" and .435" usually. It would be a good idea to slug the barrel. Once you get the diameter from the slugging you will likely have to do some testing on chamber dimensions to get the OAL. Since .44-40 is a black powder cartridge and brass is less common than others, I'd expect loadings to be near .44 Special. i hate Trail Boss with a passion but others like it for mouse fart loads. I'm guessing 5 grains of Bullseye, 6 grains Unique, etc. Small doses of pistol powder. If you go heavy (220 grains or more), a light load of shotgun powder like 2400 might work. I would consult someone with specific .44 WFC loading knowledge before I trusted a modern manual. Be advised! A rifle made in the black powder era obviously can't handle much more than lower mid-level loads with smokeless.

Couple good reads on the old 44-40 reloading. I have not loaded for it but have for it's little brother the 38-40. Some fun loads we did were using pyrodex RS powder with cast bullets.

https://www.americanrifleman.org/content/handloading-the-44-40-win/
https://www.shootingtimes.com/editorial/reloading-the-44-40-for-rifles/368990

I shoot cowboy action a good load is .427 bullet 205 gr 4.5 to 5.0 of titegroup or 4.5 to 5.0 WST use this in 100 year old marlin