I use up to 24 grains of Pyrodex --- by volume as compared to Goex FFFG black powder --- in my Colt 2nd generation 1851 Navy.
I use this occasionally, as I have a fair amount of black powder on hand and I'm burning up some old stock of Pyrodex P by plinking.
I should think that your brass-framed 1851 Navy could easily take 20 grains of Pyrodex (again, by FFFG volume, not true weight).
If you don't wish to use a loose filler, such as corn meal or cream of wheat, then use two felt wads under the ball. The felt wads should be soaked in a lubricant before use.
My favorite lubricant is a home-brew of 1 part canning paraffin, 1 part mutton tallow (or lard) and 1/2 part of beeswax. All amounts are by weight. I suggest using a kitchen scale to measure 200/200/100 grams of ingredients and placing in a quart Mason jar.
Place the jar in three or four inches of boiling water. When all ingredients are melted, stir with a clean stick or disposable chopstick. Allow to cool at room temperature.
Now, get a clean tuna can. Place two heaping Tablespoons of lubricant in the can at very low heat. Just enough to melt the lubricant. Add 100 wads. Stir them into the melted lubricant until they are well-soaked. Allow to cool at room temperature. Snap a plastic, pet lid over the can and you now have a great container to take them to the range.
If the above is too much trouble, melt some Crisco and soak the wads in that.
At the range, charge each chamber. Seat the greased wad (or two, in this instance) atop the charge as a separate operation. Then seat the ball firmly on the seated wad. No lubricant is needed over the ball if you use a well-greased wad.
Do not use petroleum-based greases and lubricants. When coupled with black powder, they typically leave a hard, tar-like fouling.
Use a ball of .380 inch if you can. It will be more accurate. Such balls are sold by Warren Muzzleloading of Arkansas at
www.warrenmuzzleloading.comRemember, all Colts typically shoot high. At 25 yards, they often shoot 6 to 8 inches high, slighly lower at 50 yards. Most shoot dead-on at 75 to 100 yards. So, at the 25 yard range you'll have to aim below the intended target.
Ensure the wedge is tight in your revolver. Use a nylon-faced hammer or small mallet to tap it in or out, or you'll mar the wedge and area around it.
The use of two, well-greased wads should leave your bore very clean. I use one wad over 24 grains of FFFG black powder so you should have room for two wads if you use 20 grains equivalent.