Well, with the grease you have a barrier to any moisture getting to the sear. With bullets the moly powder with no grease or anything else is bonded to a clean bullet by a ride in a vibrating "tumbler" with steel shot. Sorta hammered to fit the bullet metal so precisely it sticks on its own.
I've never had rust in a barrel used with moly though the climate here isn't conducive to rust anyway.
