all the bombers, missiles, do we really need bigger guns?

Kinetic energy penetrators are not as lethal as explosive heads, otherwise we wouldn't evolve from using iron balls to modern warheads.

Although the rail gun can fire a projectile at 200 miles, an active propelled missile can fly farther that that.

I don't see anything this gun can do that cannot be done by weapons already in use. One thing I noticed that with the huge size the rail gun may not be able to effectively aim at any target. I guess that's why the Navy is interested in, because they can probably mount it in a huge ship and then just steer the ship.