The Pope did not write that letter. It is not, therefore, the "Pope's definition" of WMDs. It was written by a Pontifical Council, which is purely advisory. It's also not discussing firearms in the context of legally owned firearms in the USA, but in the context of smuggling small arms illegally (eg. into the hands of Mexican drug lords or al-Qaeda).
Ok, my apologies on that one..... but,
Here is what the current pope has to say on the issue. From Pope Benedict's blog page...
The Benedict Blog: Pope Benedict Roundup!
So, then what's a conventional weapon?. . . I wish to urge the international community to make a global commitment on security. A joint effort on the part of States to implement all the obligations undertaken and to prevent terrorists from gaining access to weapons of mass destruction would undoubtedly strengthen the nuclear non-proliferation regime and make it more effective. I welcome the agreement reached on the dismantling of North Korea's nuclear weapons programme, and I encourage the adoption of suitable measures for the reduction of conventional weapons and for dealing with the humanitarian problems caused by cluster munitions
Well, as defined by Wikipedia...
"Conventional weapons include small arms and light weapons..."
Conventional weapon - Wikipedia, the free encyclopedia