When the National Rifle Association conceived the law known as “stand your ground,” the intention was to protect those who shoot others in self-defense from prosecution. The hugely consequential law, which has transformed how self-defense shooting cases are treated in two dozen states, came in response to a single incident in which the defendant was never actually prosecuted.

In 2004, in Pensacola, Florida, a 77-year-old man named James Workman mistook for an intruder a drunken government worker who had wandered onto his property after midnight. To scare the man away, Workman fired a warning shot with his .38-caliber handgun. The government employee, 35-year-old Rodney Cox, ran into Workman’s trailer, where he tried to restrain the older man with a bear hug. Cox was shot twice, and died before reaching the hospital.

After an investigation that spanned less than three months, a state prosecutor declined to bring charges against Workman. Despite the outcome, the NRA’s powerful Florida lobbyist, Marion Hammer, believed the shooter was treated unfairly, and should have been insulated from standard legal procedure — undergoing an investigation in the wake of a killing — because he was protecting himself. She crafted a bill that not only gave the explicit right to use deadly force in the face of a perceived grave threat, even when safe retreat was possible, but provided for immunity from prosecution should a person invoke the statute after shooting someone else. Later dubbed “stand your ground,” it became law in 2005.

The statute’s immunity concept was novel, and is arguably its most important aspect. It allows a defendant to call for a hearing before trial, during which he or she asks a judge for exemption from prosecution. In these pre-trial hearings, the burden is on the defendant, who must prove to a judge through a preponderance of evidence — a relatively low standard that means more likely than not — that he or she acted lawfully under “stand your ground.”

It is a special allowance under the law afforded only to defendants in self-defense cases.

“Outside of ‘stand your ground,’ I don’t know of any other defense that gives defendants immunity from prosecution,” Glenn Hess, a Florida state attorney and the president of the Florida Prosecuting Attorneys Association, told The Trace. “It’s a free bite of the apple for them.”

Now, the law’s immunity clause is on the verge of becoming even more exceptional, giving another layer of protection to those who invoke the statute after killing or wounding another person. Earlier this month, the Florida legislature passed a bill that shifted the burden of proof in “stand your ground” pre-trial hearings to the prosecution. If Governor Rick Scott signs the legislation, state attorneys will have to prove to a judge, in a “clear and convincing” manner — a legal bar just below the trial standard of “beyond a reasonable doubt” — that a defendant was not acting in self-defense. It’s a massive hurdle the prosecution must overcome just to go to trial.

“It turns 250 years of jurisprudence on its head,” Andrew Warren, another Florida state attorney, told The Trace. “The preliminary hearing already gave this additional protection to defendants claiming ‘stand your ground.’ Now we’re going several steps past that. It’s going to be much harder. There will be cases where the state cannot make the initial burden.”

“Stand your ground,” which has been linked to increases in homicide and violent behavior, has already had a profound impact on the ability to successfully prosecute cases in Florida. In 2012, a Tampa Bay Times investigation revealed that almost 70 percent of defendants who invoked the statute went free. Nearly a quarter of those cases were resolved in pre-trial hearings. That number is likely to rise should Scott sign the legislation.

“It’s essentially stacking the deck repeatedly in favor of people shooting other people,” said Mary Anne Franks, a law professor at the University of Miami.

The new bill requires the prosecution to do significantly more in order to simply bring a case to trial. Under the legislation, defendants would be tasked with providing “prima facie” evidence— a term that ultimately means “some”— showing that a “stand your ground” claim could be valid. The requirement can be satisfied through testimony given by the defendant, who might explain his or her state of mind at the time of the incident. Witness testimony or video footage would also suffice as evidence.

Once a judge agrees that a defendant has produced the required evidence, the burden shifts to, and remains with, the prosecution.

“It’s very difficult to get in someone’s head and prove what they were or weren’t thinking,” said Tamara Lave, a legal expert who has studied “stand your ground.”

State attorneys say that, even if prosecutors meet the new burden, they may be forced to tip their hand to what kind of case they will make at trial, potentially giving defense lawyers an advantage.

“The hearings will give lots of opportunities to scope out the lay of the land,” Hess said. “I can assure you the state will challenge the law at some point. Probably the first ‘stand your ground’ case that comes up. It’s going to end up in the Supreme Court.