A lot of older S&W revolvers have that rounded edge to the front of the cylinder, even for guns that never had black powder ammunition. It just looks nice, I guess, and was probably eliminated to save machining steps and thus costs.

When the Mountain Revolver first was introduced, followed by the Mountain Guns, gun magazine writers didn't refer to this as a 'black powder bevel.' They said that the rounding was to make the revolver easier to put back into a holster. Maybe I am particularly oaffish, but I could never tell any difference.