Osmium bullets with a tungsten carbide penetrating core rod
Edit : We are at space so no need to use APBC here
Osmium filled graphene : Graphene lattice holds individual osmium atoms that gives density to the otherwise too light graphene structure.
How they build this ? Well they are pretty advanced in nanotechnology, you know.
Molybdenium hulls doesnt makes sense, you can have molybdenium alloys with steel, but pure molybdenium is not usefull
Well, the game says collapsed molybdenium wich can mean a kind of neutron star-like material.
Now we are talking business.
You can have collapsed molybdenium bullets too, this is like shooting neutron stars around, messy thing...