The gun industry suffered a potentially significant legal setback Thursday when the Connecticut Supreme Court said a leading maker of AR-15 rifles could be held legally responsible for marketing practices that allegedly made the semiautomatic gun the weapon of choice for mass shooters.

In a 4-3 ruling, the court overturned the dismissal of a wrongful-death lawsuit brought by families of victims killed in 2012 at Sandy Hook Elementary School in Newtown, Conn. The suit was brought against Remington Outdoor Co., the maker of...