No gun nut, but why would a maker of something be liable for how some random customer uses it?
Are kitchen knife makers liable when someone is stabbed using one of their knives? Are van manufacturers liable when some loon decided to mow down pedestrians on a bridge?
Pretty much anything can be used as a weapon; that doesn't make the manufacturer responsible when used as such. Even weapon manufacturers.
Want to blame someone for ease of gun availability in the US? Blame successive US governments and legislators for allowing it to continue.