You have the same rifle as do I.
Note folding Marbles tand sight and folding Marbles barrel sight:
When I bought it new several years ago, I bought a box each of several top brand .45-70 300 grain jacketed bullets and a few brands of factory .45-90 ammo and the shot them all for accuracy and velocity.
All the .45-90 was of the cowboy action type of 300 and 500 grain bullets and shot OK but at modest velocity; due to reduced powder loads, they were not consistant unless the rifle was pointed up and then back down to get consistant powder position in case - fun, but not accurate or good hunting loads, so I shot them up for practice.
All the .45-70 ammo shot well and was quite accurate (much like .38 special ammo in a .357 mag revolver). I liked the Winchester Supreme 300 grain Nosler partition best and have found them very accurate and devastating on game.
My reason for wanting this particular 1886 rifle was to handload heavy loads for large and dangerous game. Many loads were tried and so far, my favorite heavy load is the 450 grain Kodiak heavy jacketed bullet at 2150 fps used for a shoot through knockdown on the running bison shown below:Though this bullet has knocked down an elephant with a brain shot, it did not penetrate the brain and only NF or Punch bullets will be used for ele in the future. With the 300 grain Nosler protected point ar 2200 fps, it has taken leopard and other thin skin game quite handily.
This is my favorite heavy rifle followed closely by my Miroku made 1895 .405 WCF light heavy.