Oscar Pistorius' ex-girlfriend told his trial he once fired his gun from his car sunroof after becoming angry with police.
An emotional day in court then climaxed with testimony from a security guard shocked to find Mr Pistorius carrying a dying Reeva Steenkamp.
The athlete had earlier told him "everything is fine" when he rang him after shots were heard.
Mr Pistorius denies both murdering Ms Steenkamp and shooting from his car.
Early in her testimony, former girlfriend Samantha Taylor broke down while discussing two break-ups with the athlete.She said Mr Pistorius had twice been unfaithful, with the relationship finally ending when he "cheated" on her with Reeva Steenkamp.
The defence pointed to emails it said proved the relationship with Ms Taylor was already over by the time Mr Pistorius began seeing Ms Steenkamp, but Ms Taylor insisted the relationship was not "officially" over when he started dating Ms Steenkamp.
The model also said the alleged shooting incident happened when he became angry after a police officer stopped him for speeding, saw the gun on the car seat and told him it could not be left there.
She accepted that he laughed around the time he actually fired the gun.
Ms Taylor testified that Mr Pistorius kept his gun "on him all the time," and described him as a man who could get very angry.
'Too shocked to help' :
Pieter Baba, a security guard working at Oscar Pistorius' gated community, the Silverwoods Country Estate, followed Ms Taylor in the witness box.
He said the athlete told him everything was "fine" when he called to investigate neighbours' reports of gunshots on the night Reeva Steenkamp was killed.
He said he made the phone call from outside the runner's villa, before dawn. His said Mr Pistorius then called him back, crying but not saying anything, before the line went dead.
When he saw Mr Pistorius carrying a dying Reeva Steenkamp down the stairs, Mr Baba said he was initially too shocked to help.
Mr Pistorius looked at the floor while Pieter Baba gave his evidence, at one point appearing to cry when the security guard described the bloody scene he found.
Crucial testimony :
The BBC's Nomsa Maseko says that while Samantha Taylor was brought in to testify in connection with the lesser firearm charge, her evidence about the alleged shooting incident is crucial to the prosecution's case over the murder charge too, as it paints Mr Pistorius as an angry, reckless gun owner.
The prosecution sought to do the same with evidence given on Wednesday, when the court heard from boxer Kevin Lerena about another incident in which Mr Pistorius is alleged to have fired a gun - in a restaurant - after it was passed to him by another friend in the group. He said Mr Pistorius asked the gun's owner to take the blame.
Towards the end of Samantha Taylor's evidence, the court was adjourned a second time, when she broke down again in the witness box.
After returning, she was asked by the prosecution if there had been other occasions when Mr Pistorius thought there was an intruder in his house.
She replied that it had happened at least twice. Crucially, she said he always woke her up before taking his gun with him to check.
Mr Pistorius claims he shot Ms Steenkamp after mistaking her for an intruder, and had not attempted to wake her before picking up his gun.
If found guilty, the 27-year-old double amputee, dubbed the "blade runner", could face life imprisonment.
Neighbours' testimony :
Friday's trial began with more testimony from neighbour Johan Stipp, who on Thursday told the court he found Mr Pistorius praying over Ms Steenkamp's body as she lay dying.
Questions again focused on his and other witnesses' testimony about the timing of various screams and gunshots.
The prosecutor insisted the sounds witnesses reported do not fit Mr Pistorius' version of events, while the defence tried to draw out differences in the evidence given by the neighbours who have appeared in court so far.
Mr Pistorius again appeared distraught as the events of 14 February 2013 were recounted.
The state is seeking to convince the court that Mr Pistorius and Ms Steenkamp, a model, reality TV star and law graduate, had an argument before the athlete fired the shots that killed his girlfriend.
There are no juries at trials in South Africa, and his fate will ultimately be decided by Judge Thokozile Masipa.