(IAAL, but this is greatly simplified -- consult a licensed attorney.)
You cannot sue Mercedes if someone decides to use a car they manufactured to ram into people on the sidewalk. Well, Maybe you can sue, but you can absolutely not win.
On the same line of logic, you absolutely can not sue Walmart if your relative gets stabbed by a knife they sold, Had manufactured, or similar.
And then along the same line of logic, you cannot sue a gun manufacturer/dealer if someone decides to use a gun they sold/manufactured to harm someone.
Guns dont kill, people kill.
IF however a gun manufacturer ran advertising suggesting people buy their guns to shoot up farmers markets, schools etc. Or if gun sellers ran advertising with subliminal messages urging people to whack their neighbor, well then you absolutely CAN sue them, and would probably have a good case.
More similarly to this case here, IF a gun manufacturer made a gun that is obviously defective, leading to injury/death, and you had a leaker come and show evidence they knew about it, and didnt care, well then you would probably have a good case aswell.
Yeah, I see someone brainwashed and agendaed here and is not me. BTW if that phrase had any hint of truth to it people could just buy nuclear weapons as long as they can afford it, but no, we know that is too much power to be handed to the capriciousness of a single random person, but they pretend it's any different with handguns; manufacturers and everyone else involved know the next victim will be some kid they don't know and therefore don't care about and doesn't concern them at all, while nuclear warfare is much ore likely to affect them directly so it cannot be abstracted away as a statistic.
yes, I have an agenda, its called freedom and anti-violence. You dictating I cannot have a gun is violence against me. Same as if I tried to dictate you cannot have a car
but yes, it is violence. owning another human being would be too, and you for sure are also aware that it is not even comparable to own a piece of metal/plastic/wood and a human being.
Gun control isn't perfect, but it does a pretty good job in most of the world. You can't let perfection be the enemy of improvement.
Naturally if my estate sues you personally, your defense would be that you didn't know the product was dangerous. Perhaps if the whistleblower's leak is widely publicized, that would weaken your defense.
Certainly the leak should weaken the maker's ability to claim they didn't know.