Also, NEVER use steel wool on a stainless or aluminum gun.
Tiny particles of the steel will embed into the metal and will rust later, ruining the surface.
The only practical way to restore a bead blasted gun is to have it bead blasted again.
As another "watch-out' make VERY sure the bead blaster is one that's been used to blast ONLY stainless or aluminum, OR clean it out and re-fill it with new blast media.
If you use a blaster that's been used to blast carbon steel, again, tiny particles of the carbon steel will be embedded into the metal and will rust.
These is one way to "sort of" touch up a small scratch on a bead blasted surface and that's to lay a piece of wet or dry metal type sand cloth on the area and tap it with a small hammer. After every tap, lift and move the cloth to keep a fresh part on the area.
This will mimic bead blasting to a lesser extent, just enough to make a scratch less noticeable,.