Pretoria, South Africa (CNN) -- One of the most respected ballistics experts in South Africa disputed key elements of the prosecution case against Oscar Pistorius on Friday, saying that the girlfriend he killed last year did not appear to have been in a defensive position when he shot her.
Tom Wolmarans, a former police officer, suggested that Reeva Steenkamp did not have her hand over her head when Pistorius fired four bullets at her.
"The left hand cannot have been against her head" because there were no wounds and no brain tissue on the inside of her hand, he said.
His analysis runs against one of the most dramatic parts of the prosecution case, when a police ballistics expert cowered with his hands over his head, imitating the position he thought Steenkamp was in when she was killed.
Pistorius, 27, is accused of murdering Steenkamp, a model, reality TV star and law school graduate. She was 29 when she was killed.
He admits that he fired four bullets through a closed door in his house, killing her, but says he thought he was protecting himself from a burglar.
Prosecutor Gerrie Nel has argued aggressively that Pistorius argued with Steenkamp before killing her. If she was crouched defensively when she was killed, that could support Nel's case, implying that she knew what was coming.
If the judge believes Wolmarans, on the other hand, it could suggest Steenkamp was taken by surprise.
Nel battered Wolmarans as the cross-examination began, demanding to know how many different versions of his analysis he gave the defense team, whom he consulted about it and whether he had changed any of his report.
Flustered and occasionally appearing angry, Wolmarans said he had fixed some of the English in his report -- it not being his native language -- but not his conclusions.
He also said he would not have changed his conclusions based on conversations he had over beer with another defense expert, Roger Dixon, because Dixon is not a ballistics expert.
Nel pounced, demanding whether the court should also disregard Dixon's testimony.
Once the questioning turned to the specifics of the night Steenkamp was killed, Wolmarans recovered his composure and stood his ground.
Nel disputed the former police officer's description of where Steenkamp was when the final bullet hit her in the head.
The prosecutor said that if her position was as Wolmarans described it, there were have been no space for her head. The ballistics expert disagreed.
He also stepped into the reconstruction of Pistorius' bathroom in the court, to demonstrate, leading to one of the few light moments in the trial.
He closed the door as he stepped into the toilet cubicle and then said to the judge, "I'm sorry, my lady, you can't see me," prompting laughter in court.
Wolmarans earlier agreed with prosecution witnesses who said Pistorius, a double amputee, was on his stumps when he shot Steenkamp. Before the murder trial began, the state had said Pistorius put on his prosthetic legs before killing his girlfriend -- implying that the killing was premeditated -- but Nel shifted his position when the trial began.
Wolmarans is in court for a second day.
On Thursday, defense witnesses included probation officer and social worker Yvette van Schalkwyk, who testified that Pistorius was not faking the dramatic suffering he has shown in court.
The gripping trial has seen Pistorius break down repeatedly, crying, wailing and sometimes throwing up as the court sees and hears evidence about Steenkamp's death.
Evidence has included graphic photos of the wounds; testimony from neighbors, friends, police and pathologists; and the actual door through which Pistorius fired four hollow-tipped bullets on the fateful night.
There is no dispute that Pistorius shot and killed Steenkamp in his home early on the morning of Valentine's Day 2013.
Prosecutor Nel tore into Pistorius over five days in court in April, saying the Paralympic medalist had argued with Steenkamp and killed her on purpose. He tried to force Pistorius to look at a picture of Steenkamp's head after the shooting, accused Pistorius of being selfish and possessive, and said he refused to take responsibility for his actions.
Judge Thokozile Masipa must decide whether Pistorius genuinely made a terrible mistake or whether he murdered Steenkamp intentionally.
Lead defense lawyer Barry Roux has said he expects to conclude his case on Tuesday.
Pistorius himself testified for seven days in April.
The defense team is seeking to cast doubt on the state's case and needs only to show there is a reasonable doubt that Pistorius meant to kill Steenkamp.
Its case will be followed by closing arguments. Masipa will decide the verdict in collaboration with two experts called assessors. South Africa does not have jury trials.
If Pistorius is found guilty of premeditated murder, he faces life in prison. He could be convicted of the lesser charge of culpable homicide, which would leave his sentence at the discretion of the judge.
The trial has gripped South Africa and sports fans worldwide who considered Pistorius a symbol of triumph over physical adversity.
His disabled lower legs were amputated when he was a baby, but he went on to achieve global fame as the "Blade Runner," winning numerous Paralympic gold medals on the carbon-fiber blades that gave him his nickname. He also competed against able-bodied runners at the Olympics.
Only those in the courtroom saw Pistorius on the stand, because he chose not to testify on camera. His testimony could be heard in an audio feed.