Firearms expert witness
Am I translating this correctly?
"There was a report submitted by a firearms expert witness".
Habia un informe sometido por un perito de armas de fuego"
A mí me suena mejor; " Había un informe sometido por un forense de armas de fuego."
to me this sounds better, There was a report submitted by a forensic firearms expert. (forense =scientist).
Perito is proficient which only covers the expert part. Go with Joe's, he is one of Massachusetts finest.
That seems accurate, except you're missing witness.
Witness is "el testigo", but it seems extraneous in your sentence.