system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why would somebody sue the gun manufacturer after a mass shooting?

Unless the gun manufacturer sold the weapons to a known criminal how in anyway are they liable for what some crazy would do? You wouldn't sue Budwieser or ford if a drunk driver hit you. So why would you sue the manufacturer  of a gun if some crazy used it?
gpt
You have the right of it with your mindset.  Unfortunately, in the 1990s and early 2000's, anti-gun groups filed tons of law suits against the gun manufacturers and retailers on behalf of gun violence victims, with the express purpose to try and bankrupt the manufacturers and retailers.

Even knowing they wouldn't win the suits, they kept piling them up, one after another, after another, to make the manufacturers and retailers lose money paying for lawyers, expert testimony, investigative staff, and the like.

That is the reason the Gun Manufacturer and Retailer law went into place, to block these frivolous law suits.  The lawsuits bankrupted small retailers, and made the potential cost of entering the gun manufacturing business too high to be worth it unless you were already a huge company.

Many people on here are trying to justify it, by saying things like "it will make them act more responsibly" and so on, but the truth is, the manufacturers are already acting responsibly.  They do not sell to individuals in almost all cases, only to retailers, who are licensed dealers or wholesellers.  They make their items to meet all standards and regulations.  If they don't meet standards and regulations, they can be sued for negligence, as any other business can.

Retailers are where it gets a little more dicey.  People want to be able to blame them for selling a person a gun if that gun is later used in a crime.  The issue is, if the retailer did everything by the book, did the background check, and so on, then they legally shouldn't be held responsible.  If they didn't do their jobs, then the law shielding them from a lawsuit doesn't protect them, and they can be taken down by the ATF for failure to follow the law.