Saying he was "not a hired gun for the defence," a blood spatter expert testified Thursday that an RCMP constable could not have shot Ian Bush in the way he has described.

Const. Paul Koester earlier told the inquest he shot Bush in self defence while the two were struggling in the interview room of an RCMP detachment in Houston, B.C.

He described Bush choking him from behind on a couch when he drew his gun and fatally shot Bush in the back of the head on Oct. 29, 2005.

Joe Slemko, who analyzes blood patterns, said that would have been impossible and used a graphic photo of Bush's body to explain his theory.

He testified Thursday that blood on the wall and couch show it would have been impossible for Koester to have shot Bush while being underneath him. There was also no evidence the body had been moved so Koester could free himself.

"From the school of common sense Bush's body position, as seen in the photograph, is very natural and indicates that he probably simply collapsed into that position," Slemko told the court.

"It is my opinion the version of Mr. Bush on Const. Koester's back choking him is not possible. Mr. Bush had to be under him."

Slemko also testified that cuts on the back of Bush's head appear to have been from the front of Koester's pistol, suggesting the gun was pointed directly at Bush, rather than at an angle.

Bush, a 22-year-old millworker, was arrested for having an open beer in public and giving police a false name. He was shot an hour later.

His family hired Slemko to refute the RCMP's version of the incident. But the expert maintained he was only after the truth.

"I'm proud to say I have an unbiased resume," he said. "The truth is what I seek.

"I'm not a hired gun for the Crown; I'm not a hired gun for the defence."

Slemko, an officer with the Edmonton police force, also explained the Bush family had only paid for his trip to Houston, not for his expertise.

RCMP lawyer David Butcher noted that Slemko was never able to examine the scene of Bush's shooting first-hand, and also missed DNA evidence of blood on Bush's hand.

He also pointed out that Slemko no longer works as a blood pattern specialist, but is instead a constable on general duty.

Slemko said he choose to work as a constable because he has not been guaranteed a position back in the ident-forensics unit.

With a report by CTV British Columbia's Dag Sharman and files from The Canadian Press