Where did he get the gun from? If it was legally purchased, then track it from there. Heavier regulations could call for that person being equally responsible for the crime he committed.
I'm sorry man, but your logic is incredibly weak.
If someone steals your car and crashes, should the car owner be liable also? Should we hold brewers or car manufactures responsible for drunk drivers? What about knife manufacturers? What if you gave me a ride to my bank, then I robbed bank without your knowledge, what percentage of the blame should we lay on you? Maybe even hold the GPS company liable for helping get us there.
Here's a crazy idea, lets stop trying to cast blame on all sorts of people, other than the actual person committing the crime. Lets not blame the inanimate object, lets blame the person committing the crime. Lets not restrict or make criminals of 99% of people or companies who did nothing wrong.
Why do people insist on treating guns differently than everything else? Some will say because guns are lethal, but when I point out that cars actually are more lethal, that fact gets dismissed. Are you calling for more restrictions on cars and knives also? If not, why?