More than three years after the killing spree inside Sandy Hook Elementary School, a critical hearing in a Connecticut court Monday will determine whether some victims can press a case against the manufacturer and sellers of the assault rifle Adam Lanza wielded to carry out the murders.

If the lawsuit brought by nine families of those killed at Sandy Hook and one surviving school administrator is successful, gun manufacturers nationwide might face strong incentives not to mass-produce and market assault-style weapons to the general public.

But first the victims must overcome the serious legal barrier erected by Congress in 2005 when it enacted broad immunity for gun manufacturers with the Protection of Lawful Commerce in Arms Act (PLCAA)—a bill that has become a major issue in the Democratic primary contest.

While that law has stopped many similar cases from going forward, the Sandy Hook families are trying a novel legal approach that centers on a violation of Connecticut state law against dangerous and deceptive marketing as well as the “negligent entrustment” of assault weapons to the civilian population. Bushmaster (which since 2012 has been absorbed into Freedom Group Inc.) is named in the suit, as is Camfour, a gun distributor in Massachusetts, and Riverview Gun Sales, a gun shop in Connecticut where Nancy Lanza bought the Bushmaster AR-15 model XM15-E2S her son later used in the killings.

Lawyers for the Sandy Hook families have already achieved one major legal victory. Gun-industry lawyers frequently try to move similar cases to federal courts, which have uniformly refused to permit suits against gun manufacturers for criminal actions of third parties. Bushmaster’s lawyers tried that maneuver last year, but in October a federal judge threw the case back to state court.

In Bridgeport Superior Court on Monday, lawyers for Bushmaster and the other defendants will argue the lawsuit should be dismissed because of PLCAA. The plaintiffs, represented by the Connecticut firm Koskoff, Koskoff & Bieder, will cite an exemption in the legislation for “negligent entrustment” of firearms, and an exemption for lawsuits where “a manufacturer or seller of a qualified product knowingly violated a State or Federal statute applicable to the sale or marketing of the product.”

The essence of the families’ case is that Bushmaster and the companies who distributed and sold the XM15-E2S had ample reason to know that flooding the civilian market with assault rifles carried unreasonable risks.

Their court filing details how AR-15s were first designed by the military to inflict maximum damage in close combat in a short amount of time, and how military and police forces that primarily use AR-15s enact extensive training and safety measures.

The suit describes in detail how Bushmaster marketed its AR-15s in military terms, with overt allusions to mass killing power. (“When you need to perform under pressure, Bushmaster delivers,” read one catalogue cited in the brief. “Forces of opposition, bow down. You are single-handedly outnumbered,” read another).

The families also cite several mass killings before Sandy Hook where AR-15s were used. “The most chilling legacy of the entrustment of AR-15s to the general population may be that Americans are no longer shocked when combat weapons are used to kill people as they work, shop, commute, attend school, and otherwise go about their lives. We may be horrified, saddened, even sickened, but we can no longer be shocked,” lawyers wrote in their filing.

“[T]he Bushmaster defendants knew, or should have known, that the sale of assault rifles, including the XM15-E2S, to the civilian market posed an unreasonable and egregious risk of physical injury for others,” said the filing. ““[T]he Bushmaster defendants knew, or should have known, of the unreasonably high risk that the XM15-E2S would be used in a mass shooting to inflict maximum casualties before law enforcement was able to intervene.”

This appears to be the first time a lawsuit used the PLCAA exemption that companies should have known, as a basic fact, that certain weapons are a danger to others even if sold legally. Lanza fired 154 bullets in 264 seconds, killing 20 first-graders and six administrators, according to a state investigation of the massacre.

If Judge Barbara Bellis agrees with the plaintiffs that the case should proceed—a decision not expected for a few months—that in itself would be a monumental development, because the discovery phase would begin immediately. That means senior Bushmaster executives would be exposed to sworn depositions, and lawyers for the families could force discovery of Bushmaster’s marketing practices and their specific knowledge about the lethality of their weapons.

Even if the lawsuit is later decided unfavorably for the families, what comes out in that discovery process could be a monumental development in the debate over whether military-style weapons should be so easily available nationwide.