Expanded background checks may have been defeated last month in the Senate, but one area of bipartisan gun-control consensus is gathering steam in American cities: tougher sentences, including mandatory minimums, for illegal firearm possession.

Harsh proposals, backed by both gun-control advocates and Second Amendment absolutists, have been introduced in Illinois and Pennsylvania, home to the violence-plagued cities of Chicago and Philadelphia. Chicago Mayor Rahm Emanuel is campaigning for state legislation that would increase mandatory minimums for illegal gun possession from one to three years. In Pennsylvania, members of both political parties are championing a mandatory five-year-minimum sentence for any convicted felon caught carrying a gun, and now a separate bill that would apply a two-year minimum for illegal gun possession exclusively in Philadelphia.

Advocates cite New York City’s three-and-a-half-year mandatory minimum sentence as a model. “The NRA believes—rightly—that enforcing the law means prosecuting criminals to the fullest extent,” New York Mayor Michael Bloomberg wrote last year. It’s easy to see why such laws remain popular: Rampant gun violence persists, and city-dwellers want action. But a more cynical calculation is also at play. Politicians know the policies are ineffective but continue to vote for them lest they be painted as soft on crime. Indeed, research demonstrates that mandatory minimums create unequal and sometimes unjust sentences. They have also helped make the United States the most incarcerated nation on earth.

But the most basic reason for opposing mandatory sentences for gun possession is that mandatory minimums don’t work. You often hear this argument regarding drug crimes, which are, admittedly, less menacing to society than illegal gun possession. But research shows that mandatory minimums are ineffective whether they be applied to unlucky dope fiends or gun-toting corner boys.

“Mandatory penalty laws have not been credibly shown to have measurable deterrent effects for any save minor crimes such as speeding or illegal parking or for short-term effects that quickly waste away,” writes University of Minnesota law professor Michael Tonry in the 2009 study “The Mostly Unintended Effects of Mandatory Penalties: Two Centuries of Consistent Findings.”

Take Massachusetts’ 1974 Bartley-Fox Amendment, which mandated a one-year mandatory minimum for illegal gun possession. “Studies concluded that it had either no deterrent effect on the use of firearms in violent crimes or a small short-term effect that quickly disappeared,” Tonry writes. He also evaluated fifteen studies of California’s 1994 three strikes law, perhaps the most famous mandatory minimum in the nation. Only three evaluations found that it reduced crime, and three actually concluded that it increased homicide rates.

“Research over many years has shown that mandatory penalties are limited because they address severity of punishment, not certainty,” says Jeremy Haile of The Sentencing Project. “Because most people engaged in criminal activity do not expect to get caught, few think about the penalties they will face if convicted.”

“The decent thing to do would be to repeal all existing mandatory penalties and to enact no new ones,” Tonry concludes. One reason for this is that, though they are intended to bring consistency and transparency to the justice system, mandatory minimums instead create disparity and confusion. Prosecutors use the threat of draconian sentences to compel guilty pleas to lesser charges behind closed doors and nail those who want their day in court as harshly as possible. Tonry cites a 2001 U.S. Sentencing Commission report to Congress finding that 96 percent of defendants charged with mandatory minimums and convicted at trial received the full sentence, compared to just 27 percent who pled guilty to lesser charges. Some of those sentences are profoundly unjust. Take the case of Weldon Angelos, a young Salt Lake City man sentenced to 55 years in prison for selling pot to an informant while allegedly carrying a gun. The judge called the sentence “unjust, cruel and even irrational” but his hands were tied by mandatory minimums. He urged President Bush to issue a pardon.

It’s not just individual stories, like Angelos’, that indicate the illogic of mandatory minimums. It’s the aggregate picture, too. Over one-and-a-half million Americans are behind bars, and six million more are under some form of correctional supervision. A 2012 analysis by Pew Charitable Trusts found that state prison populations rose more than 700 percent between 1972 and 2011. Mandatory minimums and other policies that increased sentences and time served “have been a key driver of prison populations and costs.” Mass incarceration affects the country’s poorest neighborhoods, the same ones most harmed by gun violence in the first place.

“Gun laws passed today to stop tomorrow’s suburban school shooter,” Brennan Center for Justice’s Inimai M. Chettiar warns at The Nation, “may well end up incarcerating more generations of young inner-city black men.”

In Chicago and Philadelphia, the new mandatory minimum proposals are encountering some opposition. The Chicago Sun-Times has editorialized against the Illinois measure, arguing that “the proposed longer mandatory sentence would do little or nothing to curb Chicago’s violence. It would tie the hands of experienced judges who must be allowed to make common-sense exceptions to the rule.”

An analysis by The Chicago Reporter found that, had the currently proposed state mandatory minimums for illegal gun possession been on the books one decade ago, “Illinois taxpayers would have been responsible for, at a minimum, more than $760 million in additional incarceration costs related to gun possession convictions between 2000 and 2011.”

And while proponents of the legislation point to New York’s declining murder rate, that city’s decrease began well before the state’s mandatory minimum law was implemented in 2006, as a WBEZ report notes. Chicago’s murder rate actually increased after the state passed its current one-year mandatory minimum law in 2011.

In Pennsylvania, mandatory minimum legislation has won enthusiastic backing from Philadelphia District Attorney Seth Williams and the Philadelphia Daily News editorial board. But powerful Republican state Senator Stewart Greenleaf, a one-time tough-on-crime senator turned outspoken prison-reform advocate, has so far refused to back the bill.

“I introduced most of the mandatory sentences in Pennsylvania over the years,” he says. “We thought we would get really tough on crime, and reduce violent crime and have lower recidivism and things like that. Well, just the opposite happened … All we did was fill our prisons up and violent crime continued to go up.”

Greenleaf chairs the Senate Judiciary Committee and will try to strip any mandatory sentences from the bill. Last year’s five-year mandatory minimum proposal died in his committee. And other lawmakers might not be upset with that outcome.

“They tell me privately that they don’t want to vote for it but that they feel compelled to vote for it because of the perception that they would be soft on crime … but we’re not being soft on crime. We’re being smart on crime.”

This year, Senators Patrick Leahy and Rand Paul have introduced a bill that would allow federal judges to bypass mandatory minimums. The laudable if painfully frustrated momentum to address gun violence should not obfuscate what is now clear to Americans across the political spectrum: Mass incarceration, most certainly including mandatory minimums, is folly.