If the NRA insists gun owners are responsible, then make them responsible. Every gun in the US should be registered, and cataloged for ballistics where applicable.
Possession of any unregistered firearm, any firearm with changed serial numbers, or modified to change the ballistics on file will result in mandatory federal prison time and a loss of all future gun rights.
Any time a gun is used in the commission of any crime, the registered owner of the firearm will face the same charges as the perpetrator, and serve an identical sentence or fine if a civil judgement.
Registration is mandatory and permanent until the firearm is sold and re-registered, certified destroyed, or certified stolen by the police. Upon death the weapons must be reregistered. No time limits on liability.