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 original engraving. >> the serial number may only be a few thousandths of an inch, but the compressed metal goes a lot deeper into the metal. >> to restore the serial number, duerr applied an acid solution which only works 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. compressed being the serial number. >> 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 eight digits in the serial number. >> those restored were l3145.