A legal shield written by Congress to benefit the firearms industry is posing unexpected hurdles for parents in Newtown, Connecticut, and victims of other mass shootings who want to use the courts to hold gun makers accountable and push them to adopt stricter safety standards.

The law, approved in 2005 after intense lobbying by the National Rifle Association, grants gun companies rare protection from the kind of liability suits that have targeted many other consumer product manufacturers.

It was introduced amid a wave of lawsuits brought by city governments arguing that gun companies had created a "public nuisance" by encouraging the proliferation of weapons. Advocates for gun makers said such suits threatened to destroy the industry and imperil Americans' constitutional right to bear arms.