American gun manufacturers, like pretty much any other manufacturer, can indeed be sued if their product is defective. For example: Remington has caught a lot of heat for defective triggers in their Model 700 series rifles.

When people say American gun manufacturers can't be sued, they're talking about the PLCAA, which shields gun manufacturers from lawsuits concerning guns they made being used in crimes. The PLCAA does not prevent them from being sued for defective products.

https://en.wikipedia.org/wiki/Protection_of_Lawful_Commerce_...

https://en.wikipedia.org/wiki/Remington_Model_700#Controvers...