In December of 2014 they filed a lawsuit against the Remington Arms company, because one of their AR-15 rifles was used in the mass shooting. If you're unfamiliar with this case, you may be wondering why gun companies haven't been sued into oblivion by countless families who have lost loved ones to various violent crimes.
The reason why has to do with what's known as the Protection of Lawful Commerce in Arms Act. This law prevents gun manufacturers from being subject to frivolous lawsuits, like the one Remington is currently facing. These companies can't be held liable for the actions of their consumers, certainly no more than a car company is responsible for road rage, or an alcohol company is responsible for accidents.
The only stipulation in this law is known as "negligent entrustment." If a gun company or gun store knowingly sells a weapon to a person who is clearly insane, intoxicated, or would obviously commit a crime, then they can be held liable for any damages that person causes. These Sandy Hook families believes that the way Remington advertised this weapon, makes them liable under the negligent entrustment clause.