"Couldn't help myself" Yeah, that's what they all say.
If you are looking for a gloss finish on the grips, I recommend polyurethane. Your application includes steps such as smoothing, applying a thinned solution, more smoothing, then unthinned poly. But it is my observation that the perfect application for one project may not be perfect for another. Some testing may be in order and the finished product is well worth the effort.
By "barrel stud" I am assuming that you mean the loading lever. On an 1851, the plunger-to-loading lever interface is supposed to be loose. A little rattle is okay. It isn't going to go anywhere.
In a kit on which the steel parts still have the machine marks, I start with a file. Get all of the machine marks off. On an 1851 you can do almost the whole job with a flat file. On the 1860 you will need at least a flat and a half round.
Once you get all of the machine marks off (Or if there are no machine marks to start with as it appears on your photos,) wrap gradually increasing grit sandpaper around the file. I start with 180 and work up to 220 and then 400.
You can get a fast wheel from Harbor Freight for about forty bucks and then some cotton wheels for it. They also have a kit of various polishing rouge stubs. Once you are finished with the 400 grit paper, this fast wheel will put a mirror finish on the metal.
Use eye protection and a dust mask.
Some of the guys use rennaisance wax even on the metal parts. I think this is on the strength of the argument that it is a good neutral PH protectant.
Steel wool works pretty well also but it is better on curved contours such as on the 1860 than on flat surfaces such as the Navy. If you work at it too hard, you can blurr the nice crisp interfaces between the flats of the barrel.