A weapon entered into evidence in the murder trial involving two Australian brothers and their American friend killed in Mexico is not the murder weapon, a court has heard.

Warning: This story contains details and descriptions that some readers may find distressing.

Three Mexican men are facing trial for the 2024 murders of Jake and Callum Robinson, and Carter Rhoad.

Jesús Gerardo, known as "El Kekas", Irineo Francisco and Ángel Jesús are charged with aggravated homicide, aggravated robbery, violent robbery and grand theft auto. Their full names have been withheld for legal reasons.

The prosecution has argued that the murders occurred during a robbery gone wrong, while the defence has said there is not enough evidence to prove that theory.

The bodies of the three men, who had travelled to Mexico on a surfing trip, were found down a well in a remote coastal area of Mexico's Baja Peninsula.

A fourth body was discovered with them, but is not connected to this trial.

This week a judge-only trial in the town of Ensenada heard from forensic specialists about each man's gunshot wounds.

It was the first time the court was told this level of detail, with the autopsy results previously withheld.

The court heard that ballistics experts had determined the fatal wounds were caused by bullets from a .45-calibre gun.

Jake Robinson was also shot with one 9mm bullet but that wound was not what killed him.

All three men died from penetrating skull injuries.

Earlier in the trial, prosecutors entered a 9mm gun into evidence, telling the court it had been found in a vehicle used by Jesús Gerardo.

The court heard the gun had no serial number and is what is known as an "80 per cent gun" — an unfinished frame completed with other parts after purchase, often referred to as a ghost gun.

Jesús Gerardo's ex-girlfriend, Ari Gisell, appeared before the court earlier in the trial. She was last year sentenced to 20 years in prison for her role in the robbery that allegedly led to the killings.

She testified that the 9mm gun belonged to Jesús Gerardo.

"Gerardo always had his weapon with him; he slept with it under his pillow," she said.

She also gave testimony that Irineo Francisco was always "carrying his .45".

But the ABC understands the prosecution does not have a .45-calibre gun. The only gun presented to the court is the 9mm weapon.

Court hears about fourth body found in well

A forensic witness earlier this week told the court it was likely the victims had been shot from behind.

"I do not know the height of the shooters, but for the height of the victims — [taller] than the Mexican average — I would say they were lying down or kneeling," he said.

A defence lawyer representing Ángel Jesús questioned whether the witness had enough information to determine that.

"No, I don't," the witness replied.

The fourth man found in the well also died from a penetrating skull injury, the court heard, and the expert judged that his body was in the well for between two and four weeks.

He concluded his evidence and shook hands with the bereaved parents of Jake and Callum Robinson, who have travelled to Mexico to attend the trial.