In a revolver, as you already know, it's imperative you use soft lead, since you are seating an oversized ball into a smaller diameter chamber, plus the ball reduces in diameter a little more while passing thru the forcing cone.
In a rifle you could use steel bearings if you have a thick enough patch to fill the rifling grooves to get a good seal. Soft lead will flatten out, sometimes dramatically when hitting hide, meat and bone, resulting in more shock and damage. A harder ball will not deform, and can easier pass thru entirely while doing less tissue damage, resulting in the need to use your enhanced tracking skills to locate a deer intent in burying itself' in dense brush.
Hard lead or other hard materials are less dense than soft lead, and will tend to lose energy quicker, and can have a dramatically different point of impact.
A couple of years ago, while playing mighty grapefruit slayer, I decided to load up some .50 cal. balls cast with wheelweights. POI at 50 yards was nearly indistinguishable from soft lead, but at the 100 yard grapefruit distance, POI was at least 3" lower than with soft lead, and many grapefruit temporarily escaped with their lives, until i reloaded with soft lead balls.