Have you tried searching Turbosquid?  Set max price to zero and search in Weapons and armor. There's a Maya Mossberg 500 on the first page, a max WWII Thhompson on the second, a 3ds gunpack on th ethird page, a bunch of nicely detailed 3ds guns by BombshellDesign on the fifth, and many more. It's a mixed bag and most aren't maximized for Poser. It's probably the greatest variety of freebies available.

I have shrinkwrapped copies of actual CAD VRML weapon files if somebody would be willing to poserize them, and provide them free to the community, for the M16a4, M2, M203, M240, M4, M9 bayonet, M9 pistol, MK19, and MK64 mount. Shrinkwrapped means there are no gun part internals and a legal position has already been rendered that these files can be distributed because they only represent the external configurations of the weapons.  When converted to Obj format, these files can be 10 Mb and larger. There are no textures.

My visual indexes of Poser content are at http://www.sharecg.com/pf/rgagnon