President Barack Obama met with Senate Majority Leader Mitch McConnell and Judiciary Committee Chairman Charles Grassley in the Oval Office on Tuesday to discuss a contentious issue: his plans to replace the late Justice Antonin Scalia on the Supreme Court. The meeting was the latest round in a campaign to compel McConnell to allow the Senate to consider a nominee this year, an effort in which Obama and his aides have tried to crack Republican obstruction by invoking the wisdom of one of the party’s icons. In 1988, after two of Ronald Reagan’s nominees were rejected by the Senate, Reagan warned about the consequences for the country should the seat continue to go unfilled, saying, “Every day that passes with the Supreme Court below full strength impairs the people’s business in that crucially important body.” It’s a line that the Obama team has been echoing a lot.

The stakes for the fight over Scalia’s replacement are undeniably high. But on Second Amendment issues specifically, policymakers and courts already often found themselves operating in limbo before Scalia died — and had been since 2008. That was the year that the Supreme Court, with Scalia writing for the majority, affirmed the right to bear arms for all individuals in District of Columbia v. Heller.

While his ruling was a monumental win for gun rights advocates, Scalia also used the opinion to caution that the boundaries of the Second Amendment remain unfixed, and could only be set by decisions in future cases. “[S]ince this case represents this court’s first in-depth examination of the Second Amendment, one should not expect it to clarify the entire field,” he wrote.

In the years since, lower courts have tried to figure out how far gun regulations can go without stepping on the toes of the Second Amendment. Their conclusions have been the most significantly divided on four particular questions. And since the Supreme Court has declined to hear an overwhelming majority of the Second Amendment cases brought since Heller, the lower courts and the lawmakers who take guidance from them could spend years wrangling with those matters without any firm guidance from the Supreme Court — no matter who eventually assumes Scalia’s former seat.

How to determine whether a gun regulation is constitutional

Though Scalia’s opinion in Heller established the right for individuals to bear arms, it includes this famous caveat: “Like most rights, the right secured by the Second Amendment is not unlimited.” That sentence left open the possibility that some gun regulations could pass muster, without spelling out the criteria for constitutionality.

Judges generally rely on such tests to evaluate regulations or bans that potentially diminish fundamental rights, like the freedom of speech or religion. This kind of assessment is called strict scrutiny. The constitutional law scholar Gerald Gunther described the yardstick as “strict in theory, fatal in fact,” since most regulations cannot pass strict scrutiny, which favors the protection of a constitutional right.

A more lenient test is called intermediate scrutiny, and it gives regulations more leeway. The measurement only requires that lawmakers have an important purpose for creating a rule, and that the rule actually helps advance the government’s goals.

One of the key questions that has percolated since the Heller decision is whether gun regulations should be evaluated under strict scrutiny or intermediate scrutiny. The Fourth Circuit tackled the issue last month, when it examined Maryland’s statewide ban on assault weapons. The regulation — part of the Firearms Safety Act that then-Governor Martin O’Malley signed into law in the wake of the Sandy Hook massacre — prohibits the manufacture, purchase, or sale of 60 types of assault weapons (including the AR-15 and AK-47), as well as high-capacity magazines that carry more than 10 rounds.

In its opinion in the case, Kolbe v. Hogan, the Fourth Circuit ruled that Maryland’s law must be subjected to strict scrutiny, sending the ban back to a lower court. The judges said that the state’s ban was an affront to what Scalia called the “core” purpose of the Second Amendment: “the right of law-abiding, responsible citizens to use arms in defense of hearth and home.” As a consequence, the court found that the ban was likely unconstitutional. “We are compelled by Heller … to conclude that the burden is substantial and strict scrutiny is the applicable standard of review,” wrote Justice William Byrd Traxler, Jr.

The Kolbe ruling was the first time a circuit court had used strict scrutiny to evaluate a gun ban. The Second Circuit used intermediate scrutiny when it upheld New York and Connecticut’s assault weapons bans late last year. The D.C. Circuit issued a similar ruling on the district’s own assault weapons law around the same time. And though the Sixth Circuit instructed a lower court in Tennessee to use strict scrutiny to evaluate a federal gun prohibition for the mentally ill, that order is now being reconsidered.

The uncertainty surrounding judicial scrutiny affects all kinds of gun regulations, not just assault weapons bans. Adam Winkler, a UCLA law professor who wrote an amicus brief in Heller, points out: “Whatever your gun law is, you need a standard of review.” The Fourth Circuit’s recent ruling means no such clear standard is in place, throwing the question into the legal purgatory of the “circuit split,” when multiple courts just below the Supreme Court disagree on a key issue. The Supreme Court tends to look for such discrepancies when figuring out which cases it should hear. As Slate’s Dahlia Lithwick notes, the Kolbe decision “sets the wheels in motion for another major gun fight at the high court,” though there’s no indication that the fight will reach the justices anytime soon.

Whether assault rifles are among the guns the Supreme Court meant to protect, or fall into the category that it signaled can be regulated

A month after the 2012 Sandy Hook shooting, when a gunman equipped with a semiautomatic rifle fired 154 rounds in five minutes at a Connecticut elementary school, New York Governor Andrew Cuomo signed the SAFE Act, the state’s landmark gun policy bill. The legislation banned future sales of assault weapons and magazines that can hold more than 10 rounds. A few months later, Connecticut passed a similar law, which included a list of 183 assault weapon models that were banned by the state.

Some experts think the Heller decision allowed for the prohibition of assault-style weapons, five years before Sandy Hook happened. As the court wrote at the time, “We think that limitation is fairly supported by the historical tradition of prohibiting the carrying of ‘dangerous and unusual weapons.’”

But are AR-15s and AK-47s “unusual weapons?” In the decade leading up to the bans in New York and Connecticut, millions of these so-called black rifles had already been manufactured and sold in the U.S. The distinction is crucial, as Heller supported the right to bear arms that are in common use, or guns “typically possessed by law-abiding citizens.”

In 2013, New York and Connecticut gun rights advocates challenged their states’ assault weapons bans. Their cases ultimately wound up in the Second Circuit, which had a difficult time determining whether black rifles fit Scalia’s definition of “common use,” or should be prohibited under his classification of “dangerous weapons.” Statistical evidence was inconclusive: Black rifles are in wide circulation, but not as widely owned as handguns. They also possess more firepower than handguns, but handguns are used more frequently in crimes. Unable to answer those questions, the Second Circuit assumed — for the sake of argument — that black rifles qualified as being in common use, while ultimately holding that the bans were constitutional because assault weapons are not a typical means of self-defense. An AR-15, the Second Circuit wrote, has capacities far beyond that of a handgun, which Scalia called the “quintessential self-defense weapon.”

Since it viewed black rifles as falling outside the realm of typical self-defense weapons, the Second Circuit concluded that Connecticut and New York’s assault weapons bans did not undermine residents’ right to bear arms. The court upheld the bans this October. The Connecticut Citizens’ Defense League, a plaintiff against Connecticut’s assault weapons ban, challenged that decision last month in a petition to the Supreme Court. The group cited as support for its argument the Fourth’s Circuit’s decision to utilize strict scrutiny in striking down Maryland’s assault weapon ban.

Whether the right to bear arms includes the right to bear them in public

Heller firmly established the right to use a gun to defend one’s home. What it did not address was whether gun owners have a right to carry a weapon in public for self-defense. In 2009, this issue was brought before the California courts. The plaintiffs in Peruta v. County of San Diego argue that the county’s permitting scheme for carrying concealed weapons in public is unjust and unconstitutional. The County of San Diego requires a resident to show “good cause” — meaning they must face an unusual threat of violence, such as stalking — in order to obtain a concealed handgun license. Wanting a license for general self-defense is not enough.

The plaintiffs point out that the county’s permitting policies, coupled with California’s statewide ban on open-carry, mean that San Diego residents have no lawful way to arm themselves in public. They went on to argue that applicants who were denied licenses in San Diego because they couldn’t show “good cause” were barred from exercising a core aspect of their Second Amendment rights: namely, the right to self defense. Their case made it all the way to the Ninth Circuit, where the panel of judges assigned to the case did as their counterparts elsewhere. They looked to Heller for guidance, and came away empty-handed.

Where Scalia’s Heller opinion did address public carry, it implied that lawmakers could enact limits without violating the right to bear arms: The Second Amendment, Scalia wrote, is not a right to keep and carry any weapon whatsoever in any manner whatsoever and for whatever purpose: For instance, concealed weapons prohibitions have been upheld under the Amendment …” But overall, the Ninth Circuit justices concluded that Heller does not speak “explicitly or precisely to the scope of the Second Amendment right outside the home or to what it takes to ‘infringe’ it.”

In the absence of an unmistakable road map, the Ninth Circuit panel took a different kind of cue from Scalia, turning to historical sources. Based on those records, the circuit judges reasoned that at the time the Second Amendment was written, the word “bear” was a synonym of “carry.” Hence, the court concluded, the Second Amendment guaranteed the right to self-defense in public. In February 2014, the court declared San Diego’s “good cause” requirement unconstitutional, saying it destroyed the right to bear arms “under the guise of regulating it.”

The County asked the Ninth Circuit to reconsider its decision, which the full court did in June of last year. But the court has yet to issue a second ruling. Meanwhile, two separate cases tackle the same question. Like San Diego County, the District of Columbia requires gun owners seeking a concealed carry permit to have good reason for needing a weapon to protect themselves outside the home. And as in California, the D.C. law is now being challenged in federal court. Since some circuit courts elsewhere have found that the right to bear arms covers public carry while simultaneously stating that “good cause” permit processes are permissible, clarity on the underlying question of how much latitude state and local policymakers have in writing their concealed carry rules may remain elusive.

Whether states can impose safety standards on gun manufacturers

With several other challenges to gun laws pending in the Ninth Circuit, California has become the epicenter of Second Amendment litigation. In one of those additional cases, gun rights advocates are contesting regulations for guns made and sold in the state.

As The Trace previously reported, the production of firearms, unlike any other consumer product, is not subject to federal oversight. Some states have stepped into this regulatory void to make guns safer by requiring gun makers to test their weapons for malfunctions and to incorporate safety features. California’s scheme is the most robust, partly to protect residents against a repeat of the public safety hazard caused in the 1980s and 1990s by the so-called “Ring of Fire.” Operating just outside Los Angeles, the notorious group of gunmakers produced cheap handguns popular with criminals. The companies’ guns were 3.4 times more likely to be used in crimes than guns made by other manufacturers.

To rid the market of these weapons, California created a roster of approved handgun models in 2001. The list included guns that met the state’s performance standards — such as not being prone to malfunction — and excluded both the “Ring of Fire” weapons and some semiautomatic handguns from other manufacturers. California also requires all semiautomatic guns to be built with a mechanism that blocks firing when the magazine is removed and indicators that inform a user when the weapon is loaded. The features help prevent accidental discharges, which killed over 500 people in 2013. Finally, the state also has passed a unique microstamping law. Not only must the serial number, make, and model be etched onto the exterior of the firearm, but new firearms must also “microstamp” those figures onto shell casings when the weapon fires, which helps law enforcement trace crime guns to their original owners.

The implementation of California’s microstamping mandate has been stalled by an ongoing lawsuit called Pena v. Lindley. The plaintiffs, joined by Calguns, a California lobbying group that represents retailers with a federal firearms license, has taken its challenge to the Ninth Circuit. California’s regulatory scheme is unconstitutional, they believe, because it prevents residents from buying firearms that Heller held are in “common use.” They also argue that California’s handgun roster and the state’s extensive safety requirements took so many guns off of the California market as to make it anti-competitive, thereby limiting buyers’ choices.

The plaintiffs cite an interesting detail from Heller to make their point: The very handgun at issue in the original D.C. case — a High Standard 9-shot revolver — is not on California’s list of approved guns. In other words, California prohibits the very types of guns that Heller said individuals could own for the defense of one’s home. (Hundreds of other handgun models are on the approved gun list, however.)

The case could ultimately impact other states that have a roster of approved firearms or safety requirements. Maryland was the first to create such a roster in 1988. Massachusetts followed ten years later. After Heller struck down Washington D.C.’s handgun ban in 2008, the district based its list of approved firearms off of the rosters used in California and Massachusetts. And five other states also require firearms manufacturers to meet safety standards.

The Ninth Circuit has yet to hear arguments on the case or write its opinion, but Calguns has already said it will try to take its case to the Supreme Court.

For now, because of the broad legal territory that Scalia left uncharted in Heller, our confused, sometimes contradictory “Swiss-cheese system of firearms regulation” will persist.

[Photo: AP Photo/Manuel Balce Ceneta]