On this we agree, Marty. Suppose I inherited my great grandfather's old Colt 45, which I never intend to fire but keep as a family heirloom. Would the simple act of owning that weapon be sinful? How about if I'm a sportsman who goes duck hunting once a year and keep my shotgun under lock and key the rest of the time? And what about degrees of weaponry? Would owning a .22 be a venial sin but a .44 a mortal sin?

Yes, these are ridiculous questions, but they are exactly the kind that would inevitably come up if the Church were to go down this path. And they would be impossible to answer because sin, by definition, is always a matter of intention and not accident. 

Owning a firearm is certainly a near occasion of sin for some people, but not for most. In my opinion, there are some weapons that, for the sake of the greater good, no private citizen should be permitted to own. But that decision should be based on principles of civil law, not Church teaching.