Brass-framed revolvers are inherently weak. They should not be used with full loads.
Therefore, I would not use conical bullets in a brass-framed revolver. Conicals, by virtue of their greater weight and greater surface area that contacts the rifling, typically create higher pressures compared to lead balls. Stick with lead balls of .454 inch.
18 grains of FFFG black powder should be considered about maximum in your brass-framed 1851 Navy. Certainly no more than 20 grains.
DO NOT use Hodgdon 777 in any amount in your revolver. Hodgdon warns against its use in any and all brass-framed cap and ball revolvers. However, Pyrodex P will work fine, if you can't get black powder. Use the same volume measure with Pyrodex P as you would use with black powder. Volume, not weight.
Hornady and Speer make balls of pure lead in .454 inch. Purchase these, or .454 inch balls from a reputable gun store. Be wary of cast balls sold by individuals at gun shows, swap meets, yard sales, etc. Often, these folks cast balls from any ol' scrap lead they have on hand. The result can be a ball made of hard lead, because it was cast from wheelweights, Linotype or whatever. If possible, check the hardness of the ball. If you can easily put a line or dent in it with your thumbnail, it's soft enough.
I've been shooting cap and ball revolvers for more than 40 years. Rarely have I seen a brass-framed revolver made as well as a steel-framed one. Brass-framed revolvers are relatively inexpensive because less work in fit, finishing and materials is put into them. They make good "starters" to learn the ropes, but a good steel-framed revolver is preferred.
But hey, you can still have fun with your brass-framed revolver. Just don't exceed 18 or 20 grains of FFFG or Pyrodex P. Though the design is old, these revolvers can be amazingly accurate if well made. An accurate revolver can hit man-sized targets out to 100 yards or more. They are also deadly. Never, ever discount a cap and ball revolver as weak, a joke or a toy. They have killed millions since 1836. Treat them with respect and obey all the rules of safety.
This includes keeping powder and caps well away while shooting them, so an errant spark doesn't create a BOOM.
Wear eye and ear protection. Never let anyone stand to the side while you're shooting it, or they will likely be hit with hot gases, lead fragments, flying sparks, etc. Because black powder guns cause so many sparks, don't stand in dry grass, brush or forest while shooting them. Two summers ago, an idiot here in the northwestern Utah desert stood in hip-high dry range grass and shot his black powder rifle. The result was 30 acres burned, before it was under control.
Have fun!
