Chris,
Try these two loads.... either one will yield about 1850-1900 fps in the .45-70 with 22" Bbl.
.460"-425g WLNGC/54.0g H335/WLRP/Starline Brass
.460"-425g WLNGC/50.0g H322/WLRP/Starline Brass
These are very consistent loads with low ES figures. Also very uniform accross a wide range of temperature extremes.
Yes, they will function flawlessly through the Win/Browning 1886!
God Bless,
Marshall