The prosecution in the Oscar Pistorius murder trial challenged the credibility of a defense witness on Wednesday.<br/> <br />Prosecutor Gerrie Nell scrutinized testimony given by this man, geologist Roger Dixon.<br/> <br />Dixon had told the court he did audio tests to check if the sound of a bat hitting a locked door could sound much like a bullet being fired.<br/> <br />Pistorius is charged with shooting dead his girlfriend, Reeva Steenkamp, last year from behind a bathroom door in his Pretoria home.<br/> <br />Neighbors have testified hearing gunshots from Pistorius' home, but the defense say it was the former track star swinging a bat at the door to reach his shot girlfriend.<br/> <br />(SOUNDBITE) (English) PROSECUTOR, GERRIE NEL, SAYING:<br/> <br />"All you did was you hit the cricket bat, you hit a door with a bat."<br/> <br />(SOUNDBITE)(English) FORENSICS EXPERT, ROGER DIXON, SAYING:<br/> <br />"That is so, my lady."<br/> <br />Dixon also gave testimony on visibility during the shooting, but Nell put that, too, under scr
