For years, advocates of gun control have struggled to pierce a legal shield that protects firearms manufacturers from liability, even as gunmen in mass shootings have relied on their powerful wares to amplify the carnage of their attacks.

More than a decade ago, New York City failed in its bid to go after some gunmakers under public nuisance laws. In 2009, a case against a manufacturer associated with a series of sniper shootings in Washington, D.C., was thrown out. Then, in 2015, the family of a victim in the theater massacre in Aurora, Colo., lost a case against an ammunition dealer — and was ordered under state law to pay the dealer’s legal fees.

The vast immunity offered by a 2005 federal law shielding gun manufacturers from most liability began to give way earlier this year, when the Connecticut Supreme Court issued a milestone ruling allowing some families victimized in the 2012 Sandy Hook Elementary School shooting to sue Remington Arms and other companies over their marketing practices.

Now, building on that success, those same lawyers are opening a new front against firearm manufacturers, working with the parents of a victim in the 2017 mass shooting in Las Vegas to argue that the design of AR-15-style rifles used in the massacre violates federal law.