I worked up a few loads for my Orginal NWMP carbine this winter, and took it out yesterday to shoot. Shot at a couple of 6"X8" printer paper targets. I used some old JBA brass I had on hand, 350 gr. (458 DIAM) Meister Bullets, and IMR 4198 smokeless powder. The powder loads were: 24 gr., 25. gr, and 25.5 gr. They pretty much shot similarly at about 40 yards. They mostly impacted a little high and a little to the right. Was shooting over some logs out in the woods on land my son owns, so was wasn't intending it to be a tack driver. I will be using 25 gr. in the future.
For those overly concerned about using smokeless powder in an orginal 1876 Winchester I'll tell you what didn't happen: the firing pin didn't come flying backward and take out my eye, the receiver didn't explode and turn my expensive gun into 9 pounds of scrap iron, and the barrel didn't explode and destroy a piece of history. I believe I have as much respect for these old guns as anyone, but I do not believe they must be treated as some sort of holy relic that can not be enjoyed and used as intended.