Armor penetration has everything to do with the balistic properties of the ARMOR.. when talking about modern firearms. What makes some people think that the ability to penetrate armor increases with range is the fact that modern Kevlar armors are LESS effective against slow moving projectiles. Hence, at point blank range where projectile speeds are highest, Kevlar armor is effective.. Of course, armor penetration is ALSO effected by the cross sectional area of the bullet (which is why a .22 calibur round penetrates better than a .45 calibur round) so speed isnt the only factor to take into account.
So, for a DU Cannon in space, range is going to have no effect on armor penetration.
Now I think the MESON bLaster is the item that should do more damage at range
reason being that mesons pass right through normal matter without effecting it.. until the meson decays into a normal particle (neutron? I don't recall the exact physics involved) So, over time, more mesons in the beam decay into high velocity neutrons that DO effect normal matter when they hit the target.. longer range means more time has passed = more mass the beam hits with = more damage.
Technicaly, the meson could decay into a neutron IN THE MIDDLE OF THE SHIP and thus skip over any armor. Only a small percentage would though so most of the damage would be to the outer parts of the ship. Shields should be effective against a meson beam though.
Rob