Forget "pew pew pew," the boomsticks are here

[Cray]

Sounds as if railguns are damaged significantly each time they are fired. How can this be minimised?

"Carefully."

The Powerlabs site details a lot of design elements used to protect the rails: soft projectiles (or sabots), Teflon coatings, insulation in the right places to avoid inappropriate shorting, etc. It also seems to help to maximum conductivity and thus lower resistive heating.

Ideally, the rails will be wear-resistant, hard, and resistant to arcing erosion. A lot of that is sort of a tall order because even minimal alloying can dramatically reduce conductivity. You can get away with something oxide dispersions (hard particles mechanically mixed into the base metal without chemical mixing), or a small percentage of elements like cadmium and beryllium for copper alloys. Cadmium helps copper with arcing resistance in oxidized environments.

Also, it might not be a bad idea to try to minimize or eliminate rail-projectile contact. You can coat most of the projectile in something like Teflon and achieve electrical contact between the rails and projectile with a deliberate electrical arc. (In a vacuum, inject some gas.)

I don't know their specific techniques, but the US Navy railguns seem to be further along in wear resistance than hobbyists. They are after long-lived, low-cost weapons and barrel wear is an old problem for naval guns. (Though steel barrels don't have to worry about electrical conductivity requirements.)

In the worst case, you might try sacrificial rails. After every few rail shots, eject the rails and slide in some new ones.

Coil guns largely avoid this wear problem, but coil guns seem to be limited to 1-3km/s velocities. Railguns aren't nearly so constrained in their muzzle velocities.