Weapon conversion question.

[Gideon020]

I've been lurking for a while and this forum's given me some good ideas, but I need some help on something. One of my players has something of a mad-on for the AA-12 shotgun and the Frag-12 explosive rounds and in a moment of weakness, either that or I was high on human kindness, I finally caved.

Now I can figure out some kind of fluff about both gun and ammo being super-rare, issued only to special forces, requiring some pretty tough rolls to find the stuff to make it in the field, and so on and so forth.

The real problem is I have no idea how to stat it up beyond grabbing the stats for the USAS-12 and Explosive Frag Shells off PMulcahy's website. Should I go with that and tell the player to like it or leave it, or does anyone have any advice on how I can actually expend some effort into making some decent stats?