>> since the serial number on the shotgun had been ground off, investigators sent the gun to forensic scientist timothy duerr. he knew that scratch marks usually aren't as deep as the initial engraving. >> the serial number may only be a few thousandths of an inch, but the compressed metal goes a lot deeper into the metal. >> duerr applied an acid solution that when applied only goes at one level and goes no deeper. >> it will actually eat away and you can actually burn past the point of compression. it reacts differently between the compressed and uncompressed metal. >> the acid solution removed the top surface, leaving the bottom, or compressed metal, virtually untouched. >> upon utilizing restoration techniques on this shotgun, i was able to restore five of the