DELPHI, Ind. ― Jurors in the trial of Delphi murder suspect Richard Allen on Friday heard from a firearms expert whose findings tied Allen's gun to an unspent round found between the bodies of Abigail "Abby" Williams and Liberty "Libby" German.

Melissa Oberg, an Indiana State Police firearms examiner, testified that she found through her analysis that the unspent cartridge, more commonly known as a round, had been cycled through the same Sig Sauer, Model P226, .40-caliber handgun that Allen owns. Oberg, who's been analyzing firearms for 17 years, was the only witness in a long day of tedious and technical testimony that delved into the minutiae of firearm cartridges and what causes markings on them.

▶ Friday,Oct. 25: Delphi jurors hear from firearms expert about unspent round. Follow live

Allen, 52, is facing two counts of murder and two counts of murder while kidnapping the girls. Prosecutors have alleged that Allen followed Abby and Libby as they crossed the Monon High Bridge in Delphi, threatened them with a gun and forced them into the woods, where he killed them. Allen's attorneys have maintained that Allen is an innocent man caught up in a bungled police investigation marred by a turf war between law enforcement officials.

As part of her analysis, Oberg said she cycled and fired rounds through Allen's handgun, so she could compare tool markings left after firing with the markings on the round from the crime scene. She explained that because firing causes more pressure, the markings would be more distinct.

Jurors also received a crash course on how semi-automatic weapons work, as well as what's considered a cartridge and what its parts are. Oberg also demonstrated to jurors how a semi-automatic pistol cycles cartridges through the firearm.

A cartridge is a unit of ammunition, also called a round. At its head is the projectile or the bullet. Inside the casing is the gunpowder. At the bottom is the primer, which ignites the powder and propels the bullet out of the barrel of the gun. And cycling involves pulling the gun's slide, allowing a round to feed into the chamber.

The trial comes more than seven years since Abby and Libby disappeared while traversing the Monon High Bridge trail on Feb. 13, 2017. They were found the following day, bloodied with their throats slashed and lying in the woods near Deer Creek, just south of the Delphi Cemetery. Allen was arrested on Oct. 26, 2022, more than five years later.

During a long cross-examination of Oberg, defense attorney Bradley Rozzi questioned whether the field of firearm examination is trusted science, saying other scientists are critical of the industry.

"Would you ever tell a law enforcement officer in this case that firearms testing, just like paternity testing, is 99% accurate?" Rozzi asked.

"No," said Oberg, who also testified that the error rate in the firearm testing industry is about 2% and she's not aware of ever making identification mistakes in her many years of analyzing firearms.

Rozzi also emphasized that the result of Oberg's analysis was based on examining and comparing fired ― not unfired ― rounds.

Testimony resumes Saturday, which is the second-year anniversary of Allen's arrest. The prosecution is expected to call a DNA expert to the stand. The trial is expected to conclude in mid-November.