Former San Francisco Police Sgt. Nelson Lum was a firearms instructor and is an expert on the use of deadly force.
"If the officer senses there was potential danger, he or she may draw the weapon," Lum said.
But the regulations on firing a weapon are much stricter.
"The officer may use deadly force in defense of his or her life or in defense of someone else's life," Lum said.
Officers may also fire on someone who is trying to escape, if that person committed a serious crime.
The make of a police sidearm can vary from department to department, but they are all semi-automatic handguns.
"There are no safety locks on the weapon that are being issued," Lum said.
To reduce the chance of an accidental discharge, officers are taught to place their trigger finger on the side of the gun. To fire, they press the trigger, which requires more effort than one might think.
BART video taken by a rider shows two BART officers trying to subdue Oscar Grant on the ground. It appears to show Officer Johannes Mehserle drawing his weapon as he gets up and then firing it, hitting Grant.
Former FBI agent Rick Smith is now a security consultant; he thinks the shooting was accidental.
"The officer thought he was reaching for his Taser and wasn't prepared to shoot, the gun went off," Smith said.
The video appears to show the other officer moving away seconds before the shot. Smith believes Mehserle may have told him he was going to use his Taser.
"There was something that happened that caused that officer to back away at the same time the officer shot him," Smith said.
Many departments insist that officers wear Taser guns on the opposite side of the gun belt from where their sidearm is holstered to prevent such accidents.
But again, ABC7 does not know exactly what happened because BART and its police did not return calls Tuesday.