MILWAUKEE — At 18, Julius Burton could not legally buy a gun. So he paid a 21-year-old acquaintance $40 to accompany him to Badger Guns, just outside the city limits here, and to be the official buyer of a weapon.
Mr. Burton pointed to a Taurus semiautomatic pistol and said, "That's the one I want," according to surveillance video from that day in 2009 and trial testimony. Then he helped his friend, who was struggling to fill out a two-page form.
A hovering store clerk helped as well, showing the friend how to correct mistakes and ensure he was listed as the buyer.
A month later, on June 9, 2009, Mr. Burton shot two police officers with the Taurus.
One lost an eye and was left with brain damage; the other was seriously wounded in the face.
It was a classic straw purchase, an important way guns enter the underground market, though an unusually well-documented one because of the video and the quick identification of the true buyer.
But the case is unusual in another way. The wounded officers and the city of Milwaukee brought a civil lawsuit in state court against the gun store, charging that the clerk knew or should have known that the transaction was illegal. Closing arguments are expected on Monday in Milwaukee County Circuit Court, in what has been a rarity in the last decade — a jury trial on the obligations of gun dealers who make questionable sales.
A victory for the officers, legal experts say, could bring renewed energy to civil litigation aimed at making the gun industry safer.
The case is part of a new wave of lawsuits — at least 10 are percolating around the country — that focus on gun shops like Badger and accuse them of knowingly permitting illegal sales or of being grossly negligent.
Lawsuits against gun sellers and makers were sharply restricted 10 years ago, when Congress passed a law giving the industry wide immunity from blame for the misuse of its products.
"If the jury in Milwaukee rules for the victims, it would be a notable and unusual victory," said Timothy D. Lytton, an expert in torts law and gun cases at the Georgia State University College of Law.
"It may well embolden more plaintiffs to bring lawsuits," he said, "and give new momentum to a litigation campaign that looked all but dead after 2005."
The Badger Guns clerk, Donald R. Flora, and the store owner, Adam J. Allan, say they did not realize that Mr. Burton's acquaintance, Jacob Collins, was a straw buyer, and their lawyers have cited the stringent limits on liability in the 2005 law, the Protection of Lawful Commerce in Arms Act, in defense.
In the 1980s and after, victims of gun violence and dozens of cities tried to sue weapons dealers and manufacturers, arguing that they should help pay for medical and other costs tied to their products. Many of the cases were rejected by the courts, though a few were settled.
In some cases, pressure from lawsuits appeared to prod companies into altering practices; some stopped production, for example, of cheap handguns known as Saturday Night Specials.
But the industry successfully pushed for restrictive new laws in more than 30 states, and then in Washington, to protect gun companies from liability for criminal or reckless actions by buyers over which, the companies asserted, they had no control.
Lawrence G. Keane, senior vice president and general counsel of the National Shooting Sports Foundation, the firearms industry's trade association, based in Newtown, Conn., said the law was needed to "stop suits for criminal misuse of lawful products." He noted that it includes exceptions for knowingly illegal sales or those made despite clear indications of danger, among other criteria that could permit lawsuits.
He also noted that the association, in cooperation with the federal authorities, has since 2000 run a widely praised campaign to prevent straw sales, including the promotion of guidelines for the scrutiny of customers that are followed by a vast majority of dealers.
Mr. Keane said he had not studied the Badger Guns case, but he disagreed that a victory for the victims would have wider repercussions, saying, "It's one dealer based on one set of facts."
But Jonathan E. Lowy, director of the legal action project at the Brady Center to Prevent Gun Violence, which has been a driving force in gun litigation, said the 2005 law had often derailed legitimate lawsuits, robbing victims of basic rights.
"Making gun dealers pay some of the foreseeable social costs of their products will change the way they do business," he said, just as civil judgments made automobiles safer.
The 2005 act "creates unprecedented immunity that no other industry in America enjoys," Mr. Lowy said.
The Brady Center and other groups have argued, without success so far, that the act is unconstitutional.
The high legal bar faced in the case against Badger Guns is illustrated by a verdict in June, in the only other lawsuit against a gun dealer in the past decade to reach a jury.
In Juneau, Alaska, relatives of a shooting victim sued a dealer, Ray Coxe, for making what they described as a clearly negligent if not deliberately illegal sale in 2006 to a methamphetamine-using fugitive.
The clerk left the would-be buyer unattended with a rifle; he walked out of the store with it, leaving $200 on the counter in payment, and later used it to shoot and kill a 26-year-old man.
Mr. Coxe, whose firearms license was revoked this year by the federal authorities because of repeated violations, said that it was a theft and that he bore no responsibility. The jury agreed.
Relatives of victims in the Sandy Hook Elementary School massacre in Newtown in 2012 have also brought a lawsuit against the maker and seller of the Bushmaster assault-style rifle Adam Lanza used to kill 26 people.
But their legal theory is more sweeping than the one in Milwaukee. The plaintiffs argue that they deserve compensation because the gun manufacturers and sellers should have expected that the sale to untrained civilians of a military-style weapon, with a large magazine of high-power cartridges that can be fired in rapid succession, would sometimes result in murders and mishaps.
Legal experts say the lawsuit, still in the early stages, will face high obstacles in view of the 2005 law, which was aimed at preventing this kind of claim for a weapon that is legally and widely sold.
Badger Guns, in the village of West Milwaukee, had a long history of legal violations, official warnings and machinations to prevent the revocation of its firearms license, as The Milwaukee Journal Sentinel has described in a series of reports. A second lawsuit against the store is pending in Milwaukee, also involving a weapon that was used to shoot police officers.
At the time of the sale to Mr. Burton, the store was owned by Mr. Allan, a defendant in the lawsuit. Over the years, the store changed its name and formal ownership — transforming in 2007, for example to Badger Guns, owned by Mr. Allan, from Badger Outdoors, owned by Mr. Allan's father.
On the streets of Milwaukee, Mr. Burton said in testimony from prison, where he is serving 80 years for attempted murder, "everyone knew" that Badger was the place to get a gun. In 2011, two years after the sale to Mr. Burton, the Bureau of Alcohol, Tobacco, Firearms and Explosives rescinded Badger's license.
In 2012, a brother of Mr. Allan, Mike Allan, reopened the store as Brew City Shooter's Supply, initially featuring a firing range. The store later received a license to sell weapons. The new owner has said that, to avoid problem sales, he will sell guns only to people who take out memberships in the range for $25 and demonstrate knowledge of gun safety.