There are two areas to cover here. Cleaning an old gun which has lots of grease and dirt on it and cleaning your prized possession after shooting it.
The first, Cleaning your rifle / musket, after shooting is covered very capably by two web sites and Bill Curtis does the this subject far more justice than I can on the 'Research Press' web site.
Another, which is very clear, well written with good photos can be found at www.cleburnes-division.com...personally I use hot water or windscreen wash rather than any other concoction - read the article and you'll understand.
Secondly cleaning an old and dirty gun, such as those from Nepal cashe, can be very rewarding and fun. However please, please do not attempt to 'clean' up your rifle / musket unless you are sure you know what you are doing or have spoken to someone at your local muzzle loading club about what you are intending to do, as you could ruin a potentially valuable antique. For instance a Model 4 P53 in good original and shootable condition will retail for about £1,200...an over 'cleaned' version will cost considerably less....... Also just because it may have brass on it does not mean you should polish it. Old brass develops a patina which shows the age of the gun, if you need to polish something go clean your shoes!
If you do decide to use some of my advice I cannot be held responsible for what you are doing. However below are a few pages which may help or be of interest: