(CNN) As disgraced former police officer Richard Wince awaited sentencing last year for illegal gun dealing, his lawyer told the judge Wince had already learned a hard lesson and asked that his client receive probation.

The prosecutor, however, pressed for prison time. Wince—as a cop—should have known better than anyone the risks associated with his black-market gun sales, he argued.

But there was one issue on which both sides could agree: The law under which Wince was about to be sentenced that day in a federal courtroom in Virginia was badly flawed.

It's "unconstitutionally vague," the defense attorney said.

"I'm not a big fan of the statute," the prosecutor conceded.

Both men were referring to the Firearms Owners' Protection Act, a decades-old federal statute originally proposed by the National Rifle Association that dictates who is required to have a license to sell guns, and who is not.

The law's fuzziness on that distinction and its provision that defendants must willfully violate the law in order to face prosecution, has frustrated efforts by law enforcement to combat the problem of unlicensed gun dealing in America, a CNN investigation found.

Unlicensed dealers are a go-to source of firearms for criminals and others who can't pass the sort of background checks that licensed dealers are required to perform, according to a review of dozens of court cases and interviews with federal agents and prosecutors across the country.

Guns sold by unlicensed dealers often turn up in the hands of convicted felons, at crime scenes, and in police investigations, including cases of armed robbery and murder, CNN found. Some dealers continued their illegal sales even after receiving warnings from the Bureau of Alcohol, Tobacco, Firearms and Explosives.

Mary and Terry Hunt, farmers in Washington state, and Richard Wince, a former Washington, D.C., police officer, were charged with unlicensed gun dealing.

Last year a gun sold by an unlicensed dealer was used in the slaying of an off-duty police commander in Chicago. In 2017, a gun allegedly sold by a suspected illegal dealer in Nevada to another individual was used in the fatal shooting of a California sheriff's deputy and the wounding of two California Highway Patrol officers.

"The crime they're committing is a white-collar crime," Houston Police Chief Art Acevedo said of unlicensed dealers, in a recent interview. "But the crimes they're facilitating are violent crimes that impact our communities across the country every day."

Despite the impact of their illicit sales, criminal filings against unlicensed dealers represent a small fraction of federal firearms prosecutions and receive comparatively little attention in the national debate on guns. Even successful prosecutions sometimes result in wrist-slap punishments because defendants have no previous criminal record, CNN found.

A common refrain from Second Amendment advocates is that the country does not need any new gun control laws; it just needs to enforce the ones already on the books. But the Firearms Owners' Protection Act is widely seen by those in federal law enforcement as more of a hindrance than a tool, according to current and former ATF agents and prosecutors interviewed by CNN.

"It's a huge problem," said Jill Snyder, a recently retired supervisory agent with ATF.

The impact of unlicensed gun dealing would be greatly reduced if every gun sale -- not just those by licensed dealers -- required a background check on the prospective buyer, law enforcement officials told CNN.

That's the goal of a bill passed by the House last month. But the prospect of such a measure passing the Republican-controlled Senate, where the NRA holds significant sway, is slim, political observers say.

Engaged in the business

Laws regarding the purchase and possession of firearms vary from state to state . State and local jurisdictions can enact and prosecute laws that are more restrictive than federal law, but not less so. When it comes to dealing firearms, federal law requires that all dealers, regardless of the state they live in, be licensed by the ATF. CNN focused on the federal law because it is the regulation that applies to all gun dealers throughout the country.

Under federal law, licensed dealers are required to conduct background checks on prospective buyers to ensure they don't sell to convicted felons, domestic abusers or people who are mentally unstable.

They are also required to maintain records on weapons they acquire and sell, in part, to provide a clear path for law enforcement to follow when they recover a gun that is used in a crime.

But an ordinary citizen who decides to sell a weapon or weapons, but does not fit the definition of a dealer, faces no such requirements under federal law. And a fundamental problem with the current law, according to critics, is that what constitutes a dealer is not all that clear-cut.

The statute says anyone "engaged in the business" of selling firearms requires a license. "Engaged in the business" means devoting "time, attention and labor to dealing in firearms as a regular course of trade or business with the principal objective of livelihood and profit through the repetitive purchase and resale of firearms."

The law states that it does not apply to someone engaged in only occasional gun sales or collectors or hobbyists adding to, or selling from, their personal collection.

The law places no set number of transactions that differentiates between a person engaged in the business and one who makes "occasional sales."

Assistant U.S. attorney Peter S. Duffey, who successfully prosecuted Wince, criticized the law last year at the ex-officer's sentencing hearing.

"I don't understand why they couldn't set a number," Duffey told U.S. District Court Judge M. Hannah Lauck.

The law that sought to define gun dealers was enacted in 1986 after complaints by the gun lobby that the national Gun Control Act, passed two decades earlier following the assassinations of President John F. Kennedy, Attorney Gen. Robert Kennedy and Martin Luther King, Jr., was overly broad.

That law required anyone "engaged in the business" to obtain a license, but offered no further definition on what that meant.

Firearms advocates accused the ATF of using the law to go after occasional gun sellers who weren't really dealers and to, literally, make federal cases out of innocent paperwork violations.

Jennifer Baker, a spokeswoman for the National Rifle Association, defended the current law, which she said is sufficiently clear to prosecute people for dealing guns without a license. Setting a fixed number of transactions to distinguish between a casual seller and a dealer is arbitrary, she said, and "doesn't work."

The fact that some unlicensed sellers do get prosecuted demonstrates that the law is effective when applied, Baker said.

She rejected the phrase "unlicensed gun dealer" as a "politically charged term."

"There's no such thing as unlicensed gun dealers," Baker said. "They're called felons. If you're breaking the law, you should be prosecuted and punished."

Thomas Chittum, a veteran ATF supervisor who is also an attorney, said that's much easier said than done.

Thomas Chittum, deputy assistant director of field operations for ATF, said unlicensed gun dealing cases are "the most challenging prosecution that ATF has."

He said the vague wording in the statute and the requirement that defendants be shown to have willfully violated the law make such cases difficult enough. On top of that, he said, many unlicensed dealing suspects have no previous criminal record, making them sympathetic to juries and unappealing to prosecutors.

"In my view, they're the most challenging prosecution that ATF has," he said.

Because unlicensed dealers typically do not document their sales, Chittum said, it is impossible to quantify how many guns pass through their hands each year. Whatever the number is, he said, "it's not small."

The ATF routinely warns suspected unlicensed dealers that they may be breaking the law. They do so by sending out so-called "cease and desist" letters instructing recipients to stop selling guns until they've obtained a federal firearms license.

CNN filed a Freedom of Information Act request with the bureau in June seeking, among other things, the number of such letters sent per year in recent years.

The ATF had not provided those statistics prior to publication of this story.

Hidden in plain sight

They are hidden in plain sight, peddling weapons on the internet, at gun shows, in parking lots, from the trunks of their cars. Some boast of hassle-free "no paper work" deals to set themselves apart from their licensed competitors.

They are in some cases the most unlikely of suspects: Police officers. Federal agents. A fire department captain.

Their ranks include an aerospace engineer with a government security clearance, members of a prominent farming family , and retirees who have lived a lifetime free of previous brushes with the law.

These defendants were among those in more than 50 cases reviewed by CNN involving allegations of unlicensed dealing and related offenses.

In some cases, suspects made clear that they were aware of the risks associated with black-market gun sales, but chose to ignore them.

"The [ATF] is really watching us," a suspect in Wisconsin told a purported gun buyer he didn't realize was an undercover agent, according to an affidavit filed in federal court. "I'm nervous as shit."

"As far as you know, you didn't buy it from me," the suspect added.

In Maryland last year, a suspected dealer sold an undercover agent an illegal machine gun without a serial number, according federal court records.

"You can get in more trouble for having this right here than you can for selling crack to six-year-olds," he said of the automatic rifle, which he later sold for $750 cash.

Often, when unlicensed dealing suspects are confronted by the ATF they deny they are dealers. Rather, they claim to be a collector or "hobbyist" who is not required to obtain a license.

That's what Wince, the former Washington, DC police officer, told ATF agents. He later pleaded guilty and was sentenced to a year in prison.

It's also the argument retired mechanic Paul D. Boltz tried to make when agents served a search warrant on his home in Ohio. He did so, despite the fact that he had sold more than 1,800 guns "which generated over $694,000 in gross income," according to court records.

"The defendant was involved in selling a large quantity of weapons, over time, without any background check whatsoever," the prosecutor wrote in a sentencing memorandum. "Ultimately, this activity presents a significant risk to the public."

Boltz, then 65, pleaded guilty in 2017 to dealing firearms without a license, a felony, and surrendered 136 guns to authorities. He received no prison time. Instead, he was sentenced to five years of probation and fined $40,000. Attempts to reach Boltz through his attorney were unsuccessful.

There is no indication in Boltz's court file that guns he sold were later linked to crimes. But, often, authorities say, when unlicensed dealers sell a high volume of weapons without determining who they're selling to, it's only a matter of time.

One egregious example involved an aerospace engineer in Virginia who authorities said sold about 200 guns to a crack dealer who in turn sold them to various people, including some he knew were barred from possessing weapons. Scores of guns linked to the two men have been recovered by police in Washington, DC and five East Coast states. Guns linked to the drug dealer have been tied to three different homicides, including that of the dealer's own cousin, according to court records. The crack dealer was sentenced to 12 years in prison. The aerospace engineer got 18 months.

On the West Coast, more than a dozen firearms sold by members of a farming family in Washington state turned up at crime scenes or in police investigations scattered around the San Francisco Bay Area, according to authorities. Each of the family members was charged with felony unlicensed gun dealing, but they pleaded guilty to a lesser misdemeanor offense and were sentenced to probation.

One reason cases against unlicensed dealers sometimes involve otherwise sympathetic suspects is that would-be dealers benefit from a felony-free record that allows them to continually buy guns to later resell, according to court records and interviews with agents.

They obtain their stock of weapons from licensed sellers who properly conduct background checks prior to the purchase. The unlicensed dealers then re-sell the guns at a profit without performing any background check or filling out the required paperwork documenting the sale, agents say.

Recent cases of alleged unlicensed gun dealing around the country include charges filed earlier this year against a supervisor with U.S. Customs and Border Protection in Southern California. Authorities arrested Wei "George" Xu after seizing an arsenal of more than 250 guns from his Los Angeles-area home, including AK-47 and AR-15-style rifles and Uzi-type sub-machine guns, according to prosecutors. Xu has pleaded not guilty and is awaiting trial. His lawyer did not return a call seeking comment.

Two other Southern California police officers have been charged in a separate case and a third, a former lieutenant, was sentenced to a year in prison for unlicensed dealing last month. Other recent cases have involved a former fire department captain in Kansas City, Mo and former supervisors with the Border Patrol and DEA in Arizona.

Acevedo, the Houston police chief who also serves as president of the Major Cities Chiefs Association, was particularly critical of fellow law enforcement officers who engage in illegal gun dealing.

Houston Police Chief Art Acevedo

"Shame on them," Acevedo said. "They're the ones who should know more than anyone that firearms are ending up in the wrong hands."

Because defendants in unlicensed gun dealing cases must be shown to have willfully violated the law, their cases sometimes begin with a warning letter from the ATF that serves to put them on notice that they are at risk of prosecution.

Kenneth Cherry received three warnings, according to court records -- one from the ATF, one from the licensed dealer who was legally selling him guns and one from his own brother, who works in law enforcement.

Still, five years after the initial warning from the ATF in 2012, Cherry continued selling guns at flea markets in Georgia and Tennessee.

In 2017, the ATF sent an undercover agent to attempt to buy a weapon from Cherry at his home in northern Georgia. Sitting in the living room were two metal gun safes, fully stocked with dozens of weapons. In the drawer of a nearby coffee table was a blue book on the value of guns, according to an affidavit filed by an ATF agent.

Cherry acknowledged to the agents having been given an application for a gun dealer's license years earlier, but said he didn't want one. "He did not like the government telling him to whom he could and could not sell firearms," court documents state.

Cherry's defense attorney wrote in a sentencing memo to the judge that his 70-year-old client had spent his entire life as "a law-abiding and productive citizen" and was "deeply upset about his situation, and has felt that his poor judgement has jeopardized his ability to assist his family."

Cherry's punishment? Three years on probation -- with the first six months spent in home confinement.

His attorney declined comment. Attempts to reach Cherry himself were unsuccessful.

In the case against former DEA supervisory agent Joseph Michael Gill in Arizona, the prosecutor told the judge in a sentencing memorandum that Gill was "selling large numbers of firearms to whoever would purchase them," including "a member of a drug trafficking organization."

Assistant US Atty. Phillip N. Smith said Gill betrayed his oath and undermined the DEA's mission "all for personal profit."

"Individuals who illegally sell firearms," Smith wrote, "pose a clear and present danger to society as a whole."

The prosecutor implored U.S. District Judge Raner C. Collins to send a message by putting the disgraced agent behind bars.

Gill's defense attorney asked the judge for a sentence of probation, citing his client's "model career in law enforcement" and "otherwise spotless record."

The judge granted his request.

He sentenced Gill to five years of probation for felony unlicensed gun dealing and fined him $15,000. Like Cherry, he was to spend the first six months in home confinement, "at the discretion of the probation officer." Gill declined comment through his attorney.

Operating with impunity

Most cases of unlicensed dealing not only go unprosecuted, but undetected, said Snyder, who led the San Francisco Field Division of the ATF until she retired last year.

"Unfortunately, we work everything backwards with these cases," she said. "There's no interest in a case unless a gun actually gets used in a violent crime."

Some black-market dealers were able to operate with apparent impunity for years.

John Stewart Davis, an Army veteran living in Montana, sold guns illegally for decades before he was finally caught. When agents raided his home in 2015, they seized 354 weapons, including multiple machine guns and a "Street Sweeper" shotgun, according to court records.

During the investigation Davis allegedly boasted to undercover agents about selling "$200,000 worth of stuff" at a gun show in Spokane,Washington, according to court documents. He suggested that one of the undercovers "pay him extra since there was no paperwork."

The investigation also unearthed a possible explanation for his longevity in the illegal gun trade: Davis obliterated the serial numbers from two handguns and a rifle he sold one agent so the guns couldn't be traced, according to court records.

He also gave the purported buyer a cover story.

"If you do get caught with 'em," the 71-year-old gunsmith said at the time, according to court records, "say you bought them [off] ...some real young guy in a parking lot of a gun show."

At Davis' sentencing hearing in October 2016, the prosecutor sought a prison term of at least eight years based on his conduct, including obliterating serial numbers and illegally possessing short-barrel shotguns, both of which called for enhancements under federal sentencing guidelines.

"Davis sold any type of firearm, including machineguns, to anybody with the cash to pay for it," assistant US attorney Paulette L. Stewart wrote in a sentencing memorandum to the judge.

Davis' defense attorney asked the judge for a sentence of probation. He characterized his client as an old school craftsman who built and sold weapons based on a "handshake and a look in the eye."

Only recently, after learning from the ATF that one of his guns "ended up in the possession of somebody that wasn't a good guy," had he come to appreciate why gun laws exist, the lawyer said.

Davis then spoke briefly on his own behalf.

"Now I'm starting to realize, yeah, I was wrong," he told U.S. District Court Judge Dana L. Christensen.

The judge noted that Davis once had a federal firearms license back in the 1980s and asked why he let it lapse.

"I'm just not good at jumping through hoops and doing all the paperwork," he said.

He also asked the defendant why he'd obliterated the serial numbers on the guns he sold to the undercover ATF agent.

"That was probably to protect myself," Davis said. "I thought they might misuse it, maybe, or get caught with something that wasn't right."

Christensen said he was mindful of Davis' military service, his clean record, and his obligations as a family man.

"All of those matters are on the front of my mind and make this, quite frankly, a difficult case for me," the judge said shortly before imposing sentence.

The problem, he said, was the parade of drug defendants he either read about or sentenced in his court on a daily basis "who possess firearms, many of them purchased from individuals like Mr. Davis at gun shows or at gunsmith shops, and many of these firearms have obliterated serial numbers."

It "defies belief," the judge said, that Davis only recently realized that drug dealers were buying his guns. "These same individuals shoot and kill law enforcement officers," Christensen added.

He then sentenced Davis to four years in federal prison -- less than half the time the sentencing guidelines recommended.