Saying otherwise is just dishonest - or straight up deliberately ignorant.
Two things really got to me: First was Gus's mom being totally unable to reconcile her thoughts to the title of my post. She insists on carrying the guilt with her forever because (imo) she can't help but blame herself for "improper handling of a firearm". If only she had pointed the muzzle to the ground; if only she'd made sure Gus had stayed on the horse; if only etc etc etc
Second is Gus's dad saying he's fearful of government involvement in product liability issues because (paraphrasing) "they'll make the gun so safe it won't work".
A string of cynical calculations plus the fear and the careerism on the parts of company employees made it inevitable for these "accidents" to happen, and certainly Mrs Barber shoulda been more careful, but it was a gun that killed Gus.