You've identified several problems that are common with 3D scanning projects. Yes, shiny metal parts are difficult for optical scanners to handle, since reflections cause erroneous points to be recorded. It's common to use dulling sprays, white paint, or even plaster casts of the object to overcome this.

Any scan, to be useful, will need some post-processing to remove extraneous points, fill holes, and register different scans with one another. Some scanners come with software that can do these things, like the Capture/Wrap bundle we sell. But most don't, so you need to get the software separately. Fortunately, there are starting to be some free and cheap packages that do some of this.

If your gun is no more than 2.4" in thickness, you could use a Roland MDX 20 to scan it. It uses a touchprobe to scan with, so the reflective surface isn't a problem. It has all the resolution you'd need for a project like this, down to .002" granularity. And it doubles as a mill, so you can use it to carve the recess as well. These were discontinued by Roland, but I still have one left in stock.

Andrew Werby
www.computersculpture.com