DELPHI, Ind. — The judge in the Delphi murders trial has ruled that the defense team cannot have a metallurgist take the stand and testify against the State's findings that a bullet cartridge found at the scene matches suspect Richard Allen's gun. Allen faces four counts of murder in connection to the brutal Feb. 2017 killings of teenagers Abby Williams and Libby German near the Monon High Bridge in Delphi. He was arrested in October 2022 by Indiana State Police - over five years after the girls' tragic deaths. One of the prosecution team's key arguments against Allen is that an unspent round from his Sig Sauer P226 handgun was found between the girls' dead bodies. The bullet and its origins are a crucial component in the case. Attorneys representing Allen had previously asked to bring on Dr. William Tobin as a witness to testify and call into question the State's theory that the bullet came from Allen's gun. On Wednesday, Special Judge Fran Gull ruled that Dr. Tobin's testimony is inadmissible and that he may not take the stand. RELATED: Understanding ‘unspent bullet’ evidence in Delphi murders "Dr. William Tobin is an expert in the field of metallurgy and has been recognized as an expert in that field by several courts of various jurisdictions," Judge Gull wrote. "Dr. Tobin is not firearms expert, has had no training in firearms identification, and has never conducted firearms examination." Gull - a special judge assigned to the Carroll County case out of Allen County - specifically concluded that Tobin could not testify because he never examined the evidence in this specific case. She wrote that, because of this, his testimony "lacks relevance." The defense team has long pushed against the science that can tie tool markings on the cartridge to a specific gun. In addition to this ruling, Wednesday also marked the fifth day of testimony in the Delphi murders trial. For more information on today's court proceedings, click here.