The second ammendment grants us the right to bear arms, however no one disagrees that there are people who should never be allowed to posses any weapons.
The weapon liability rule would make it the responsibility for anyone selling ANY knife, sword or gun to be responsible for damages done by said weapon. Sellers would be allowed to be exempt for amounts of damages provided they had liability insurance. Buyers could be exempt for surcharges provided they underwent examininations to determine sanity or posses their own liability insurance which would in turn require mental health examination.
The weapons liability rules goes on to include "unsecured" weapons in that the owner of a weapon that gets stolen or used without the owners knowledge is ALSO liable for all damages done.