investigators sent the gun to forensic scientist timothy duerr. he knew that scratch markings usually aren't as deep as the initial engraving. >> the serial number may only be a few hundredths of an inch, but the compressed metal goes a lot deeper into the metal. >> an acid solution was applied which 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 the techniques on this shotgun, i was able to restore five of the eight digits in the serial number. >> those restored were l3145. but there was no shotgun with those serial numbers registered