We can’t even make a rail gun work reliably here on earth. And more to the point what valuable metals? The astronauts didn’t see any valuable metals lying around on the lunar surface. They found rocks and dirt very similar to what you’d find in Hawaii. Nothing that you’d get anything valuable from
“Railgun” should read “Linear Motor”, the broader category that includes railgun.
http://en.wikipedia.org/wiki/Linear_motor
Common lunar minerals
Calcium (Ca)
Aluminium (Al)
Iron (Fe),
Magnesium (Mg)
Silicon (Si)
Titanium (Ti)
Oxygen (O)
The value of these is that they are common industrial raw materials and they are NOT at the bottom of the earth’s gravity well. They may be used to fabricate space vehicles & habitat without having to pay the HUGE cost for the freight elevator ride up out of Earth’s enormous (relative to the Moon’s) gravity well.